Improve Audio Bus button pressed state visibility

[ckaiser]

Light theme audio bus buttons have an issue where I believe thanks to how the icons are loaded, they're much darker by default, which means it's a lot more difficult to modulate them into being bright colors like it would be on dark themes. This caused me (and probably other users, I'm sure) to accidentally click "solo" on a bus and I being unable to figure out why audio wasn't working for a while since at a glance I could not distinguish the button's pressed state.

This PR addresses that by adding a colored underline to the buttons which keeps them as compact as before but allows you to tell at a glance which buses are solo/muted/bypassed.

Before & After comparison:

The worst offender:

[akien-mga]

I'm not against adding an underline, but I still think the discrepancy between the light and dark themes needs to be fixed properly.

[ckaiser]

Adjusted the colors (didn't realize I could just bump it up over 1.0)

New look:

[Calinou]

Looks good to me. The underline helps make it more obvious which buttons are pressed, although we could opt for a translucent background instead (like we do for active buttons in the 2D/3D editor viewport menu bar).

[ckaiser]

Looks good to me. The underline helps make it more obvious which buttons are pressed, although we could opt for a translucent background instead (like we do for active buttons in the 2D/3D editor viewport menu bar).

I went for the underline because to make a translucent background work I would've had to increase their margins and make everything a bit wider and taller which took away from the compactness of the UI, with this the size remains the same.

Maybe making it a bit chunkier wouldn't be a problem, but this was the least disruptive solution

[akien-mga]

Thanks!