If you're using  Lazarus with Qt  -  you can actually leverage Qt qss
stylesheets to get an easy way to style your user interface. In fact,
it has more features than lcl provides by default.  QSS is a css-like
language that lets you define the look of your UI,  but also lets you
set up animations, mouse-over effects, and so on.

You could include a qss file alongside your application,  but you can
also bake the qss file into your executable using resources. 

Here's how:

=================================================
Step 1: Include the qss stylesheet as a resource:
=================================================

* First, create or copy the qss file in your source folder.
* Project->Project options, Resources, click (+) Add.
  Select the qss file, should show up as a relative path.
  Set the resource type to RCDATA, give it a name.

===================================
Step 2: Load and use the qss style:
===================================

You  can do this  when you  load your application,  or before you the 
affected controls/forms/windows  -  Your main window constructor is a 
good place,  for example   -  in the example below,  I'm applying the
stylesheet to the main window, which means that you can define styles
for the main window  and any of it's children  in the qss stylesheet,
which is typically what you'd want  - but you can in theory apply the
style  to  any  control,   provided you map the LCL control  to  it's 
equivalent Qt control type.

=====================================================================
A note on control names.

At this time, LCL's Qt widget implementation does not pass the widget
name to the Qt widget.

Consequentially,  qss selectors by control  name will not work out of
the box, for instance:

QLabel#LblTitle {
  color: #0f0;
}

...will not work by default,  because qt has  no idea what control is 
named 'LblTitle', even if you set the name in lazarus.

The workaround for this is to explicitly set the name in your code,
for instance:
...
const
  TitleName: UnicodeString = 'LblTitle';
begin
  QObject_setObjectName(
    QObjectH(TQtWidget(LblTitle.Handle).Widget),
    @TitleName
  );
end;
...

Creative minds can probably come up with a way to automate this for
all controls -  but it would be even nicer if the LCL actually were
to implement this at some point in the future. In fact, all of this
is a bit of a hack,  and a formalized stylesheet interface would be
nice to have (tm). (actually, i did end up writing this code, see
way below).

=====================================================================
Example loading code:

...
uses classes, qt6, qtobjects, qtwidgets;
...
constructor TMainWindowForm.Create(AOwner: TComponent);
var
  NativeControl: TQtMainWindow;
  ResourceStream: TResourceStream;
  ResourceText: TStringList;
const
  TitleName: UnicodeString = 'LblTitle';
begin
  inherited Create(AOwner);
  Randomize;
  NativeControl := TQtMainWindow(Self.Handle);
  ResourceStream := TResourceStream.Create(HInstance, 'QTSTYLE', RT_RCDATA);
  try
    ResourceText := TStringList.Create;
    try
      ResourceText.LoadFromStream(ResourceStream);
      NativeControl.StyleSheet := unicodestring(ResourceText.Text);
    finally
      FreeAndNil(ResourceText);
    end;
  finally
    FreeAndNil(ResourceStream);
  end;
  (* Optional, set the Qt name of specific controls if needed *)
  QObject_setObjectName(
    QObjectH(TQtWidget(LblTitle.Handle).Widget),
    @TitleName
  );
end;

===========================+=========================================
LCL -> QT control mapping: |
---------------------------'

This isn't a complete list - you can figure this out by examining the
lazarus source code if you need to, but I figure I'd help ya along :)

+--------------+-----------------+----------------------------------+
| LCL Control  | Qt equivalent   | Description                      |
+--------------+-----------------+----------------------------------+
| TEdit        | QLineEdit       | Single-line text input.          |
| TMemo        | QTextEdit       | Multi-line text editor.          |
| TLabel       | -- n/a --       | Qt is not used to render these.  |
| TStaticText  | QLabel          | TLabel but with a win. handle.   |
| TButton      | QPushButton     | Standard command button.         |
| TCheckBox    | QCheckBox       | Binary toggle with text.         |
| TRadioButton | QRadioButton    | Exclusive selection toggle.      |
| TComboBox    | QComboBox       | Dropdown selection list.         |
| TListBox     | QListWidget     | Scrollable list of items.        |
| TForm        | QMainWindow     | Window (also TMainWindow)        |
| TPanel       | QFrame          | Generic container.               |
| TGroupBox    | QGroupBox       | Titled container for grouping.   |
| TScrollBox   | QScrollArea     | Container with scrollbars.       |
| TPageControl | QTabWidget      | Tabbed container control.        |
| TTabSheet    | QWidget         | Individual page/tab content area.|
| TSplitter    | QSplitterHandle | Draggable bar between panels.    |
+--------------+-----------------+----------------------------------+

+----------------+-------------------------+
| LCL Control    | Qt equivalent           |
+----------------+-------------------------+
| TProgressBar   | QProgressBar            |
| TTrackBar      | QSlider                 |
| TSpinEdit      | QSpinBox                |
| TFloatSpinEdit | QDoubleSpinBox          |
| TListView      | QTreeWidget/QListWidget |
| TTreeView      | QAbstractScrollArea     |
| TStatusBar     | QStatusBar              |
| TToolBar       | QToolBar                |
+----------------+-------------------------+
(Note: QTreeWidget is used only for TListView with ViewSTyle=vsReport
       other styles use QListWidget)

+--------------+-----------------+----------------------------------+
| LCL Control  | Qt equivalent   | Description                      |
+--------------+-----------------+----------------------------------+
| TMainMenu    | QMenuBar        | The horizontal menu at the top.  |
| TPopupMenu   | QMenu           | Right-click context menus.       |
| TSpeedButton | -- n/a --       | Qt is not used to render these.  |
| TBitBtn      | QPushButton     | Button with an icon/glyph.       |
| TScrollBar   | QScrollBar      | Standalone scroll control.       |
+--------------+-----------------+----------------------------------+

Beyond the main controls, qss also uses a concept of sub-controls.
For instance:

* QComboBox::drop-down - The little arrow button on the right of a 
                         dropdown.
* QScrollBar::handle   - The actual thing the user drags.
* QTabWidget::pane     - The border area around the tab contents.
* QProgressBar::chunk  - The actual colored "fill" of the progress 
                         bar.

========================+============================================
Example qss stylesheet: |
------------------------'

/****************
 * TPageControl *
 ****************/


QTabWidget::pane {
  border: 1px solid #639d74;
  top: -1px;
}

QTabBar::tab {
  background: #333;
  color: #83bd94;
  padding: 10px;
  border: 1px solid #639d74;
  border-top-right-radius: 12px;
}

QTabBar::tab:selected {
  background: #000;
  color: #83bd94;
  border-bottom: 0;
}

/***********************
 * TList, TMemo, TEdit *
 ***********************/

QListWidget, QTextEdit, QLineEdit {
  background: #333;
  color: #639d74;
  border-radius: 12px;
  border: 1px solid #639d74;
}

QTextEdit:focus, QLineEdit:focus {
  border: 2px solid #83bd94;
}

/***********
 * TPanel *
 ***********/

QFrame {
  border: 0;
}

/***********
 * TButton *
 ***********/

QPushButton {
    background-color: qlineargradient(
      x1:0, y1:0, x2:0, y2:1,
      stop:0 #3e3e3e, stop:1 #2d2d2d
    );
    border: 2px solid #8ba355;
    border-radius: 4px;
    color: #e0e0e0;
    font-family: serif;
    font-size: 14px;
    font-weight: normal;
    padding: 6px 15px;
}

QPushButton:hover {
    background-color: #4a4a4a;
    border: 2px solid #c5f059;
    color: #ffffff;
}

QPushButton#confirmButton {
    background-color: qlineargradient(
      x1:0, y1:0, x2:0, y2:1,
      stop:0 #2d3e2d, stop:1 #1e291e
    );
    border: 2px solid #558b55;
    color: #c8e6c9;
}

QPushButton#confirmButton:hover {
    background-color: #364f36;
    border: 2px solid #7cb37c;
    color: #ffffff;
}

QPushButton#dangerButton {
    background-color: qlineargradient(
      x1:0, y1:0, x2:0, y2:1,
      stop:0 #4e2a2a, stop:1 #331a1a
    );
    border: 2px solid #8b5555;
    color: #ffcdd2;
}

QPushButton#dangerButton:hover {
    background-color: #5f3333;
    border: 2px solid #b37c7c;
    color: #ffffff;
}

QPushButton:pressed {
    background-color: #1a1a1a;
    border-style: inset;
    padding-left: 9px;
    padding-top: 9px;
}

QPushButton:disabled {
    background-color: #252525;
    border: 2px solid #444444;
    color: #666666;
}


=====================================================================
Automatically setting control names.

As mentioned earlier, one could presumably automate the 
synchronization between Qt control names and LCL control names, so
below are some functions which do exactly that.

Getting the order of operations right can be a bit tricky at times.
Be sure all controls are fully instantiated before setting their
names. Order of operations should be:

  * Instantiate controls
  * Name controls
  * Load stylesheet.

procedure SetControlQtName(AControl: TWinControl);
var
  QtWrapper: TQtWidget;
  NativeHandle: QObjectH;
  WS: WideString;
begin
  if not AControl.HandleAllocated then AControl.HandleNeeded;
  QtWrapper := TQtWidget(AControl.Handle);
  NativeHandle := QObjectH(QtWrapper.Widget);
  WS := WideString(AControl.Name);
  QObject_setObjectName(NativeHandle, @WS);
end;

procedure SyncQtNames(Window: TWinControl);
var
  i: Integer;
  Child: TControl;
begin
  for i := 0 to Window.ControlCount - 1 do
  begin
    Child := Window.Controls[i];
    if Child is TWinControl then
    begin
      SetControlQtName(TWinControl(Child));
      SyncQtNames(TWinControl(Child));
    end;
  end;
end; 

procedure TMainWindowForm.Loaded;
begin
  inherited Loaded;
  SyncQtNames(Self);
end;