You can view the source code for the newTab.css file here:

chrome://browser/content/newtab/newTab.css

BTW, the correct file name is userContent.css - the C for Content has to be upper-case or Firefox won't use that file.

And if you aren't familiar with css, check out this website - https://userstyles.org/ - for pre-made "styles" that can be used with Stylish or be put in a userContent.css file with a little editing to make it "yours".

Thanks, I got some ideas from https://userstyles.org/ searching for about:newtab - working through them and slowly getting there! (also, thanks for the case typo)

See also:

• chrome://browser/skin/newtab/newTab.css