Add a sepia mode
Many readers prefer a sepia mode and consider it more pleasant and yielding less eye strain than reading black text on a white background or white text on a black background.
Firefox have a "reader mode" that lets you read in sepia mode.
When using sepia in the reader mode in Firefox, it uses #5b4636
for the text color and #f4ecd8
as the background color.
These are defined on line 49 in chrome://global/skin/aboutReader.css
body.sepia {
--main-background: #f4ecd8;
--main-foreground: #5b4636;
--toolbar-border: #5b4636;
}