Cannot Restore Firefox to ALL original defaults

I have been using the latest version of Firefox on my Window’s Desktop 8.1 for the past couple of years and unfortunately, I was trying to change some of the features like using different colored fonts, tried using different themes and somehow, I really screwed it all up. I have tried deleting my total account, have tried removing Firefox completely from my computer, under the set-up location, the programs and then re-downloading it several times, only for it to remain completely screwed up. It’s ALL completely whitened out backgrounds with of course the colored fonts still being the same but the pictures and everything is just this awful plain white where you cannot even see any signs of the original Firefox that I have been using. I need to use this for school but have worked at trying to restore all defaults and don’t realize what it is I need to do. Can anyone please shed some light on this, PLEASE???

Look into this:
https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings