OnshapePlus от DHMango

Create themes in the editor, choose from ready-to-use themes, or import user-created themes, and easily apply them to Onshape! Click the icon in the extension menu, and set which theme will be displayed in Onshape's "light" or "dark" mode. Reload Onshape to see them in action.

https://github.com/DHmango/onshape-plus