@bill63 @FlatPepsi is the person to talk to on that…

however, look into MMM-WeatherGraph.css for what can be changed easily (DO NOT EDIT THIS FILE) - then make the changes in custom.css

If the two files should be:
MagicMirror/modules/MMM-WeatherGraph/MMM-WeatherGraph.css
and
MagicMirror/css/custom.css

In a default pi install, that would be a directory located in your home folder.