First, a brief caveat: I offer the following information with no guarantee what implications, if any, it has upon your security. I doubt that it does, but it's prudent that I still note this.
That said, this is how you would do it within the TAILS os, The file path may be different in your specific case, but the general approach will likely be the same.
Two useful files you would consider modifying are:
I am sure that there are others.
The userChrome.css file already exists; the userContent.css is one you will need to create.
Relative to what you have already attempted, perhaps you are not using the default user profile, and as a corollary, are modifying a css file which the Tor Browser is simply not loading?
This one will modify the browser UI. With regards to eye strain, here is one setting you may consider making, which will make the URL address bar more readable:
/* set the URL bar font characteristics */
font-family: monospace !important;
font-size: 18pt !important;}
Close and save the file.
As you already know, this one will override website-specified styles. In the same folder as the userChrome.css file, create userContent.css. I am less well versed in how to modify this. Also, some of the css you specify may not necessarily be supported by the browser. However, I am testing it as I am typing this, and some (but not all) of the text can may be rendered larger with:
/* Obviate need to zoom on many sites. */
font-size: 150% !important;
line-height: normal !important;
This modification alters much of the TAILS homepage font size, but affects the apperance of StackExchange considerably less so.
For changes to take effect, close and restart your browser
If you've yet to review them, these references will also be of use to you: