How to set tab stop width in my theme

[xtal256]

Hello.

I have a custom theme, based on a really old one for Firefox 2.0, that I use to modify the Pale Moon UI to my liking.

Whilst looking at code on GitLab, I noticed that the indentation was larger than I expected. It seems that Pale Moon uses a default tab width of 8 characters for fixed-width fonts. I prefer a width of 4 characters.

Now, I found that I can set a CSS property -moz-tab-size to control this, and that the default CSS for pages is in the file resource://gre-resources/html.css.
What I cannot figure out is where to override this in my own theme extension?

I guess I could put the CSS rule anywhere (e.g. browser.css), so long as it is loaded for every page. But that seems a little messy. Is it possible to make a separate CSS file for overrides to the default html.css file?