First things first: you really need to pay attention to Andy's warning in his CSS Editor announcement: "I have not *yet* added sanity checking, so be a little careful."
Make sure you create a backup before you play around this feature.
Second, to pick a color, use a tool like this one I just picked randomly by doing an online search for "CSS colors":
https://www.quackit.com/css/css_color_codes.cfm
Move the slider up and down to pick a color space, and then move the picker around to choose the color.
Once you've found the color you want to use, make a note of the hex value, the one that starts with a hash mark.
For example, pure blue is 0000ff.
I made some notes in my article about the CSS Tool that may be helpful:
https://www.toshen.com/ke0fhs/pi-star-notes.htm#csstool
Good luck!