Improve visibility of separators in modern theme

[passivestar]

Tweaks to separator colors to help address #112258 (comment).

Separators in popup menus now use the opposite color to the popup background and the contrast of regular separators is slighly increased

Master	PR

[Chaosus]

Horizontal separators should be changed too:

[passivestar]

Horizontal separators should be changed too

Lowered and elevated separators are both equally good choices, it's only a matter of preference which ones to go with. Popup separators needed to be swapped for elevated in this PR because popups in this theme need to be extra dark to contrast with the rest of the editor, but the rest of the editor itself doesn't have that limitation and can use either.

Few examples of apps where you can spot lowered separators are rive, unreal, unity, affinity:

Here's a quick comparison in godot:

I tried using both options with the minimal theme at different stages, the reason it has lowered separators now is because in my view separators of the opposite color of clickable content make clickable content stand out more. Separators that have the same color as clickable content make toolbars look more homogenous and feel like there are more "things" in them, while lowered separators draw less attention to themselves and make the UI feel a bit lighter. If extra visual separation for content is needed I would recommend redistributing space instead (smaller gaps between content, larger gaps around separators):

Also note that it's possible that we will be making the entire UI a little brighter to improve the overall contrast to address the rest of the #112258, which in turn will also make those separators stand out a little bit extra.

In the end I'm all for going with whatever option community likes best in subjective choices like this, but I'm also aware that there will be some bias towards elevated separators due to the fact that that's what godot had previously and what the existing users are used to. So I want to help make a deliberate and unbiased decision, whatever it is

[Repiteo]

Thanks!