In V11 CE, I'm using a custom website theme I purchased in the App store and I made a change to the custom.less file in the website builder which resulted in a black screen with no accessible user interface elements.

I logged out on one computer and now I can't even access the login page, but I'm still logged in to the backend on another computer.

I can't go to the website editor and click on the 'Reset' button for the custom.less file because all I see is the black screen.

Is there some way I can replace the problem custom.less file with the default file from the theme in the backend? 

I've looked in Technical/User Interface/Views but can't find any trace of the custom.less file there.