I know this is pretty much a duplicate of How can one make firefox ignore my GTK theme entirely?, but the answers on that one are no permanent solution. It works by launching firefox from the terminal.

I would like to know a solution that works for every instance of firefox no matter how it was created. There is the possibility to edit the userContent.css file, but the settings you make randomly do not apply to some sites or in some situations, strangely, even with the !important added...

I have a dark GTK theme and this results in some textboxes having a black background with black text with a userContent.css that has

input, textarea {
    color: black !important;
    background-color: white !important;
}

Update

I changed a setting in about:config from true to false, namely browser.display.use_system_colors. Everything appears normal and well now, for one exception: everything that runs on localhost. This includes PHPMyAdmin and a website I am making. I would like to know if there is a solution to this.