Overlay: too little contrast to differentiate active panel mode

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Problem description

When the dock panel overlay mode is active on a given panel, there are a set of panel-specific modes that can be selected from the cog wheel icon:

The currently selected mode highlights the icon next to the label. However, there is too little contrast to quickly see which mode is active.

A better option instead of highlighting the icon would be to use a check mark next to the icon, which would unequivocally show the active mode.

### Full version info

```shell
OS: Ubuntu 22.04.5 LTS (ubuntu:GNOME/ubuntu/wayland)
Architecture: x86_64
Version: 1.1.0dev.42613 +8 (Git)
Build date: 2025/07/20 02:12:57
Build type: Release
Branch: bim-refactor-arch-iter-2
Hash: 5fd6fb2fbbe253ba86e1192b3a50e4f7e1f805b3
Python 3.10.12, Qt 5.15.3, Coin 4.0.0, Vtk 9.1.0, boost 1_74, Eigen3 3.4.0, PySide 5.15.2
shiboken 5.15.2, SMESH 7.7.1.0, xerces-c 3.2.3, IfcOpenShell 0.8.1.post1, OCC 7.8.1
Navigation Style/Orbit Style/Rotation Mode: Gesture/Rounded Arcball/Window center
Stylesheet/Theme/QtStyle: FreeCAD.qss/FreeCAD Light/Fusion
Logical DPI/Physical DPI/Pixel Ratio: 96/159.34/1
```

### Subproject(s) affected?

None

### Anything else?

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct