UPDATE: Since I didn’t find any other better workarounds in the last few days, I decided to polish this up and release it.
I just uploaded the extension (with the Red/Blue Safety Borders and the Dark Mode) to Gumroad for free.
I posted the full setup guide and the download link over in the Tips & Tricks section: [Unofficial Extension] High-Contrast "Black & White" Theme for n8n (for eye strain)