"To be stupid, selfish, and have good health are 3 requirements for happiness, though if stupidity is lacking, all is lost" - G.Flaubert
The only system-wide tweak available in the control panel is the "text size" which affects the display DPI
I don't think that any 2 scienctists would agree about the "optimal" color scheme for long computer work, but many users are comfortable working with dark background schemes that are not too glary. Black or blue backgrounds are customary for console windows. Unfortunately for some years now there's no (easy) way to tweak the color elements that control all windows. At least you can control colors for your xplorer² windows. Let me show you how.
xplorer² has a rich option set that among other things allows you to choose the main font and background colors. There are two colors to consider, the one for the active pane (the one you are working on) and the inactive pane colors (all remaining panels). The latter is easier to reach, just use Tools > Options menu and you will find it in the Window property page:
To get to the active pane color, you need to start the external Advanced options editor. This is in again Options menu or if you are using the Ribbon interface, you will find it under File (leftmost) ribbon tab:
Here look for the Active panel background option under Layout tab. Tick the option box and then use the [...] dots button to choose the color. While you are at it you can choose a darker color for the ribbon background.
After all the color choosing, hit on OK or Apply button, then restart xplorer², which will now be in its darkest guise — beware of the dark side! You don't need to choose a separate color for active/inactive panels, but doing so helps you recognize which one is the active pane. Also note that the toolbar backgrounds and other small parts of the window are still windows-10 white. The consolation is that they are the minority. The text colors are chosen automatically to provide a good contrast to the selected background.