Solved: System Colors Used in Firefox

[bentHnau]

I'm using the latest version of Firefox (54.0.1) and the browser colors are set to "use system colors." I like the background color (which is not an option in Firefox's selection of colors), but I want to change the font color.

What are the system settings that Firefox uses to select these colors? I assumed "system colors" had something to do with the theme I selected from my DE (XFCE) options, but changing the theme doesn't seem to affect the system colors.

I tried changing colors with userContent.css, but it doesn't seem to work—the font color I put in the file seems to be applied randomly and only when I have "use system colors" checked, and the page background color is ignored.

[debiman]

so in other words, you tried, and were 50% succesful?
you are on the right way.
gtk theme colors are usually applied to the user interface (what is around the actual web page) and somedropdown menus and such.
web site (and also empty tab etc) style can be influenced by some *.css files, yes.
this has been asked gazillions of times by windows, mac and linux users, the web is bursting with resources on this topic.
addons exist to make it easier if you don't like writing your own css.
etc. etc.
all i'll say is that many css rules require a "!important" appended.

[dasein]

so in other words, you tried, and were 50% succesful?
you are on the right way.
[snip]
all i'll say is that many css rules require a "!important" appended.

In this case, "many" translates into "required whenever one wants to have a particular .css attribute override other settings"

[bentHnau]

I used the !important flag for every CSS rule in userContent.css.

[bentHnau]

I figured it out. Applying styles to the whole web page requires the * tag, e.g.,

* {
background-color: #232323 !important;
}