I've wanted to change settings pages background in my Thunderbird themes. I've checked that they're defined by --in-content-page-background: var(--background-color-canvas); variable. Manipulating it in developer tools works, but when I try to change it in my theme source files it doesn't work. I've tried declaring value for both --in-content-page-background and --background-color-canvas as well as overwriting it by my own variable for :root, :host and even *, of course using !important parameter. :(