@leomirror there are three or four approaches

1 the module documents the classes it uses
2 the module provides a css file that uses the classes it uses
3 you read the source code of the module ( I did this)
4 you use the developers window to examine the content to get the classes
see
https://forum.magicmirror.builders/topic/14862/help-with-a-couple-css-issues?_=1680719852507

clock module does not do 1 or 2
I wasn’t near a system to do 4

if you had two instances of the clock module and wanted the styles to be different, you would use the module identifier as the key selector

#id_name1 .time { color:red; } #id_name2 .time { color:blue; }

you could find that name thru option 4

also the css thing before the { is called the selector clause, operated left to right
https://www.w3schools.com/cssref/css_selectors.php