Here are some basic styles you may want to customize (shown with their default Scalar settings):
|Page background color|
|Page text color|
|Colophon (footer) color|
|Primary navigation buttons|
|Secondary navigation buttons|
To customize other styles, use your browser's Inspector on specific page elements to identify the CSS rules applied to them, experiment with alternate values, and then paste your alternate definitions of those rules into a Custom CSS field to try them out. It may be helpful to have the both CSS field and a sample page open in separate windows while you hone in on the styles you want.
In some cases you may need to use
!important(see above) to force your custom styles to override the defaults.