Even though it already happened to you (maybe even despite you already having the following done), here's a precaution for others who may stumble into here:
If your mom saw it because of the big-lettered title of the site:
Go to preferences, and on the first page, check the hide logo box. I only noticed and did this recently, and I think it also applied to my mobile webpage too.
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Also, to make sure she doesnt recognize the site colors again, go to the bottom left corner of the site, and change the theme.
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If you got caught because of other reasons, then if youre on mac you can quickly hide your current app using cmd+h or cmd+m. I havent used windows in awhile, but i think you can minimize one window with the windows key + down arrow. Or you can hide all apps and show just the desktop with the windows key + d (looks a bit too sus). Try having like a harmless/innocent application "behind" your current window, because hiding everything looks sus (unless your parents are ok with thinking that you were on cornhub or something).
Also, going full-screen in chromium browsers hides the address bar on windows by default (apparently).
On mac, you have to uncheck this default setting:
All of these are just basic non-technical steps, but hopefully easy enough as a bare minimum for most. Theres further steps like using a browser extension to replace keywords within the site itself.