ArtStation Pro members have the ability to override the CSS styles on website themes.
Note that this feature is intended for advanced users who know how to inspect a webpage using Chrome/Firefox dev tools to get the class definitions and override them. Unfortunately, because this is such an advanced feature and every use case is different, we cannot offer much support for this feature except to help point you in the right direction. If you're scared of code, we recommend that you don't use this feature.
Also just a note, we make changes and improvements to our themes constantly. Although many of our changes likely won't break common CSS overrides, it might. You've been warned.
Generally, the workflow for doing CSS overrides is to find the style definition using the Chrome or Firefox inspector, previewing changes using the developer tools, then copying those definitions into the Custom CSS text field on ArtStation and saving the theme.
One of the powerful aspects of using Custom CSS is that you can use any font you want. You're not limited to our selection of fonts on themes. For example, you can import the Google font and then override the fonts on the website as you wish:
font-family: 'Roboto', sans-serif;