Better contrast between active and inactive tabs of stacked windows

[phil-opp]

When using stacked windows, the active tab has a lighter background than inactive tabs. However the difference and contrast is too small for my taste. I find it difficult to spot the active tab on first glance.

Unfortunately, I found no way to adjust this in the Desktop->Appearance settings. The "Window background" setting seems to affect both the active and inactive tabs. I even tried to look through some RON theme files, but didn't find anything to tweak the contrast.

I even tried enabling the high contrast accessibility settings, but this has no effect on the tab backgrounds either.

It seems like the contrast is hardcoded currently. For example, there is a hardcoded step value of 5 here:

libcosmic/cosmic-theme/src/model/theme.rs

Line 1061 in b4533e3

get_surface_color(bg_index, 5, &step_array, is_dark, &control_steps_array[1])

This should really be configurable in my opinion, or at least default to a larger value.

[mmstick]

Adjustments may require input from @maria-komarova to check if our tabs are using the right contrast.

[maria-komarova]

I need to work more on the @selected_state_color that we should apply to the selected tab background. Sadly, this is a complicated one considering all the customizations that we allow and all the surface layers that we have.
I don't think this setting should be configurable though, this is a matter of having better defaults.