Default text style for Perspective - make it smaller

Is there an easy way to reduce the size of the default Perspective font?
Will this work across all components (tables, charts, etc.)?
I know that a theme’s CSS can be edited but that will affect projects other than mine and get overwritten on upgrade.

You already have the answer, duplicate the theme folder obviously with a new name and assign it to the project.

You could also use a parent theme as well and just override the things you need like font size