When the user activates High Contrast mode with themes High Contrast #1 (yellow text and black background), High Contrast Black (white text and black backbround), then goes to browsert's preferences, General tab, Language and Appearance section, Colors button/modal and changes the "Override the colors specified by the page with your selections above" drop-down from "Only with Hich Contrast themes" to "Never", then the backgrounds would change to white which would be incompatible with light colored text colors. I believe that this is somewhat intended, but I will confirm the issue and let it be discussed further.
Bug 1688429 Comment 7 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
When the user activates High Contrast mode with themes High Contrast #1 (yellow text and black background), High Contrast Black (white text and black backbround), then goes to browsert's preferences, General tab, Language and Appearance section, Colors button/modal and changes the "Override the colors specified by the page with your selections above" drop-down from "Only with Hich Contrast themes" to "Never", then the backgrounds of webpages would change to white which would be incompatible with light-colored text colors. I believe that this is somewhat intended, but I will confirm the issue and let it be discussed further.