Hard coding settings is wrong. Respect system colors and system fonts. Suggest alternative choices. Don't just change them, especially without a way to revert to system defaults.
In Thunderbird 151.0.1 context menus changed to dark blue with white text, and subject headers also white, which stand out painfully. Previously everything respected system colors.
Accessibility, supported by fully customizable fonts and colors, is not a nicety if you are visually impaired. Wrong fonts and colors make using a product a struggle or unusable.