Make slider colors themable

[NABN00B]

Improves #18802 .

My proposed solution for the slider highlight problems.

This is an improved version of what I showed at #18802 (comment)

This implementation allows for, better customization and somewhat accurate simulation of v1.14.4 (or even the current) behavour.

What this PR does:

• combine all the colors of the slider's filled bar, knob and label into a single one.
• inside popups the slider now uses 2 new colors*
• outside popups the slider now uses the same colors as any other GUI item
• update the default themes to use the new popup slider color definitions (@hrydgard will want to adjust them though, please check)

What this PR doesn't do:

• change the slider's unfilled bar in any way
• update my custom themes to use the new popup slider color definitions (coming in a separate PR)

Note:
I only implemented 1 common color for both focused and down popup sliders because we can't directly switch from either state to the other. The same applies to other GUI elements, so I'm assuming this is intended behavour.

Screenshots of focused sliders inside popups.

[hrydgard]

Good stuff!

I will review this properly in a couple of days, sorry for the delay.

[NABN00B]

I'm now contemplating whether the knob and the label should be colored differently from the filled bar if the slider is not focused / down ... 🤔

I also forgot to handle the case when the slider isn't enabled.

[hrydgard]

Hm, well, the disabled case is not very interesting, it never happens now because we disable the button that cause the popup. I think we can leave it for later.

As for the focused, let me know if you want to change it or not. I'll test this first, but from what I can tell I'm fine with how this is turning out.

[NABN00B]

As for the focused, let me know if you want to change it or not. I'll test this first, but from what I can tell I'm fine with how this is turning out.

I'm ok with the way it is right now for 1.19, I'm worried about over-complicating things if I make further changes.

Don't forget to adjust the slider colors in Default / Dark.

[hrydgard]

OK!

I'll try to get this in this afternoon.

[hrydgard]

Hm, trying with Slate Forest:

Not sure what focused color to choose.

I sort of feel that it's the right side of the slider that should change color when focused, the light gray could change to something like the color of the popup buttons.

[NABN00B]

Hm, trying with Slate Forest:

Not sure what focused color to choose.

As I said in op I'll get the updates to my custom themes in later, because some other changes were also made.

[NABN00B]

This is how they look right now

[hrydgard]

Oh ok, those are pretty good.

[hrydgard]

I'll adjust the colors in Default/Dark in my next PR.