[Feature] Style Readability Theme Custom Background Color

[SiameseParrots]

Is your feature request related to a problem? Please describe.

I'm using Edge splitscreen, with wikipedia open the left side in immersive-reader-mode ^{_{sometimes I toggle it's read aloud option}} and on the right side another wikipedia page in stylebot-readability-mode. The fonts matches nice, thanks to type-any-font-into-the-font-option-input. That's great! Just the background makes contrasts. Since the sepia theme from immersive-reader is different to here. ^{_{also edge's sepia color is rather white, so instead I set their theme to orange}}

Describe the solution you'd like

So:
------> maybe an intensity-slider on top of the Grayscale for each theme
---------------------------------------or---------------------------------------> background-/text-color input

Additional context

this extensions helps me to stay focused on the content^{_{after I'm done with all the tweaking}}. thank you and also thank the contributors for that.