Onshape Feature Studio is based on Ace Editor, so it has support for settings, as described here.
The settings are not persistent though: every time you reload or switch Feature Studio, your preferences are lost. The inbuilt settings menu also only allows you to set font size, not font family.
This gist contains a bookmarklet you can click to quickly re-apply your preferences, including font family, and a userscript that applies them on a regular interval.