Fix FileSystem dock styles across split modes

[norepro]

Two commits for scroll hints had impact on FileSystem dock style:

• f187b8b introduced TreeSecondary for the tree pane, but only applied it in horizontal split mode.
• a45bdce removed the existing ItemListSecondary from the file list in vertical split mode.

The combination results in vertical split and tree-only not having background styles while horizontal split does.

Apply TreeSecondary and ItemListSecondary styles in all split modes. This gives consistency between the modes and with other elements.

Fixes #119359

[YeldhamDev]

There are two severe problems here:

• The single-view mode having a background breaks visual consistency with the rest of the docks.
• The horizontal split-view mode backgrounds have no margins, so they just hug the borders.

[AdriaandeJongh]

So the solution would be to just remove the backgrounds everywhere instead, right?

[YeldhamDev]

So the solution would be to just remove the backgrounds everywhere instead, right?

I'm not sure I follow? Backgrounds are hidden when scroll hints can be used. There are plenty of places where backgrounds are still used where hints don't make sense.

[AdriaandeJongh]

Like the author, I’m trying to figure out a solution to fix the inconsistency that in only one of the three modes the background appears. What do you think the solution is?

[norepro]

There are two severe problems here:

* The single-view mode having a background breaks visual consistency with the rest of the docks.

* The horizontal split-view mode backgrounds have no margins, so they just hug the borders.

Fair. This issue is more complicated than I thought... my goal was to make it self-consistent among split modes. But I am guessing way more users see the default single-view than split and,by comparison, it is the odd one out.

I also see the second point. I was focused more on the colors and usability and missed that detail.

I'll mark this PR as draft while I consider options here and bring up any ideas in the discussion thread.

[YeldhamDev]

CC @passivestar, since this is a design change, even if small.

[passivestar]

This shifts it from "in 1 of 3 modes the background appears" to "in 2 of 3 modes the background appears", if @AdriaandeJongh is satisfied with it I'm fine with it, since it's his original linked issue. I think it's fine if the goal is to get the same look for both split modes.

If I could I'd make a rulebook for when there needs to be a background and when not, but I looked at a lot of different apps when we were porting the theme to see how others handle it, and it's all case by case depending on where it makes sense, with most leaning towards no backgrounds unless there's a big benefit from them. One thing I'm sure of is that you don't want to go for full consistency here because the previous theme consistently had backgrounds in every tree and every item list and it made the UI look busier than it should have been in the main editor layout.