Feedback on the modern theme

### Tested versions

4.6 dev3

### System information

Godot v4.5.1.stable - TUXEDO OS 24.04.3 LTS on Wayland - X11 display driver, Multi-window, 2 monitors - OpenGL 3 (Compatibility) - Mesa Intel(R) Iris(R) Xe Graphics (RPL-P) - 13th Gen Intel(R) Core(TM) i7-13700H (20 threads) - 62.52 GiB memory

### Issue description

Though I do think the new theme looks nice, there are a couple of readability issues that I noticed after using it for a few days. Some of those might be deliberate choices, in which case they can be disregarded.

I'm one of those who do struggle with colors, especially in low contrasts, so these points might not look the same for everybody else as they do for me or other colorblind people.

I'm aware that the classic look is still available, and switching to it does fix those issue for me. But the idea here, is not to argue if one is better than the other, or how easy it is to switch, but rather to give feedback on the actual modern theme, so it keeps the editor appealing to new comers, but also to make it more readable for everyone who wishes to keep using it, especially those with bad eye sight.

Here are the points, in no particular order :
1 - The guidelines in the Scene and Filesystem tabs are inconsistent. Sometimes they're there, some times they are not, depending on what's selected, even if you moved away from the tab an now working on another area of the editor. In the screenshot, there are no guidelines in the Scene tree, but there is one the Filesystem tab, even if I'm not actively working on any of them. On the classic theme, the guidelines are always there, but the line is highlighted for the selected line. This makes more sense to me, as the guidelines do not pop in and out of existence. I actually though it was buggy when I was playing around with the editor. Suggestion here to pick one and roll with it, either show the guidelines, or not.

2 - The padding on the menus is way too bulky. I think a bunch of pixels could be trimmed here.

3 - There is no distinction between the menu, and the text editor here.

4 - Text and icons are way to washed off compared to their background, this is by for the most troublesome to me, given my color problem, and also the most present through out the editor. Suggestion here is to slightly lighten the text and icons towards white. I do thing there is quite some margin before it pops off too much and starts to look bad

5 - It's hard to say where the editor ends and where the animation panel begins here, though I think there is this proposal to address that : https://github.com/godotengine/godot-proposals/issues/13581

6 - Either the text in the tabs is to small, or the tabs themselves are too big. Suggestion here it to either make the text a bit bigger, bolder (better readability), or at least reduce the size of tabs a bit, which won't improve the readability, but should (subjectively) look better. Check the Blender tabs (Layout, modelling, sculpting) ...

### Steps to reproduce

N/A

### Minimal reproduction project (MRP)

N/A

The report identifies the modern theme in the Godot editor but names no source files, tests, or entry points. Start by reproducing the six readability concerns on the reported setup; done would require an agreed, scoped set of theme changes with clearer contrast, spacing, separators, and tab sizing in the affected editor areas.

Written by the indexing model from the issue text.