Add tree match background theme color

[octref]

Testing #69299:

I quite like how this used to work:

It's also consistent with IntelliSense:

The new type to filter experience has some contrast problem, as the bg color of list item leaks into bg color of selection range because of transparency).

@joaomoreno I don't know if we are committed to this visual representation of filtering, but I'd suggest something like this (full height, offer 2 them-able colors for filtering, filterSelection fg/bg). This would make it much easier to maintain contrast.

[joaomoreno]

@octref This is theme dependent. Type filter even exposes special theme keys for this. Which theme are you using?

[octref]

I'm using Nord. Which color key are you using for the match box background on the labels? I guess Nord has it set to a color with transparency.

[joaomoreno]

Hmm we're actually reusing findMatchHighlightColor for that.

[octref]

"editor.findMatchBackground": "#88c0d066" is what Nord is using. This is readable against text editor background, but it's not readable on top of tree focus/select bg.

If a tree item is selected and tree is in focus, I can't see the type-to-filter box at all:

[joaomoreno]

Then we'll just create that theme color.

[octref]

@joaomoreno Thanks! Make sure you update vscode-docs as well.

[joaomoreno]

Good call! microsoft/vscode-docs@e92eee5