Change color of the sidebars's scrollbar

**Your Godot version:** Latest documentation

**Issue description:**
Recently, a scrollbar was added (or it's color changed) to the latest documentation.
However, the color selected (a red similar to the one of the logo) really grabs the attention too much.

This color is supposed to emphasis things, that's why it make sense to use it for the titles in the sidebar. However, I don't think grabbing the attention to a scrollbar make a lot of sense.

I suggest we change the scrollbar color to something darker / less attention grabbing. Something like this maybe (but any other less emphasizing color will do :) ):


**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/latest/