As most of the info about the HTML and CSS you find is not Node-RED or dashboard specific, I'll link one more thread for you. That may be helpful to get started.
Couple of things first..
Many things can happen if you start to play with CSS. You may break whole thing, but you can make your page really good looking. But be aware that it can be addictive.
The result of your adjustments is art. So if you are changing CSS for your dashboard, you are an artist. (and a little bit a programmer of course)
I'm not an expert of art nor the CSS.
The tools
Browser developer tools. https://developers.google.com/web/tools/chrome-devtools/css The main too…