This is part of an effort to make the Mask Editor more intuitive (https://developer.blender.org/T93097).
The idea to change line widths emerged from this discussion (https://developer.blender.org/D12776).

Click me, I am a GIF:

• the line width of mask splines can be controlled using the slider in the overlay menu
• splines from the active mask layer are 2pxl wider (it is much clearer which splines are being edited)
• feather splines are not affected by the slider (default to 1pxl width)

I don't think this would be better suited for a theme, as the spline width needs more tweaking during work on masks.
Opening the preferences and adjusting it there everytime seems to be more work than benefit (what do you think?)...

Implementation:

• adds a width parameter (range 1.-5.) that controls the GPU_line_width when drawing spline lines

TODO:

• need to assign a default value of 1.f for the property