Modern Style: Use a StyleBox in signals and groups

[DeeJayLSP]

Applies the StyleBox from #112233 on signal and group subsections.

Before	After

Classic for comparison:

[YeldhamDev]

I hope we don't end up with several instances of this workaround in the code, otherwise it would be better to just deprecate the BG color element and introduce the stylebox properly in Tree.

[DeeJayLSP]

I hope we don't end up with several instances of this workaround in the code, otherwise it would be better to just deprecate the BG color element and introduce the stylebox properly in Tree.

Apart from the ones covered in this PR and the Action Map editor, there are no more than 3 places left for it to be used.

[passivestar]

Looks good although I do have a concern that they don't bring enough value to justify a bit of extra noise they add to the dock, considering they are simple trees and node/signal separation is already somewhat prominent from indentation and repeating signal icons, and groups only have 2 categories in them. The input map tree from #112233 is different because stripes help match buttons to their rows in it

It might be worth thinking if this is worth copying from the classic theme, but it's a small change so I won't die on this hill

On a sidenote it might be worth considering how this gonna go with alternating stripes if those are added to trees

[DeeJayLSP]

it would be better to just deprecate the BG color element and introduce the stylebox properly in Tree.

Just opened #112371

[DeeJayLSP]

Marking as draft until #112444 is merged in order to make things simpler.

[DeeJayLSP]

Modified so now it doesn't use the workaround anymore. Screenshots updated.

Despite this being kinda divisive, I still want to see if more people wants it

(This is probably the last place I'll push for this highlight, as stuff that used prop_subsection apart from this and the action map editor didn't look good).

Looks good although I do have a concern that they don't bring enough value to justify a bit of extra noise they add to the dock.

One could argue that the node types in the Inspector tab do have highlights (which isn't changed in the Modern theme), so this would be just spreading it to the Node tab:

(Not sure if this is a valid comparison since the tone is different)

[Repiteo]

Thanks!