Correct various Admin Centre settings which are causing sign-in and other problems

Question

I must start out by saying, I do not work as a MS365 tenant Administrator - I bear this title only because I purchased a Business Standard licence because I like to work with this software and have SharePoint, Teams, etc etc - I know very little about managing a tenant but am doing my best to learn. Mostly, I just need to get the odd issue resolved as it pops up, which also contributes to my learning which is slowly progressing!

I notice issues with dark mode keep being on - it's not local settings as if I toggle it off, it just turns back on the next time Iog in (I'm talking noticing this in the browser mostly); it's something in the tenant, can anyone guide me as to where to go and what settings to change?

Similarly, File Explorer asks me to sign in but when I try I'm told 'There's something wrong with your account' but if I log in to My Account, I've got no idea which of the settings here to look at but can already determine that much of what is here is managed at the tenant level so once again, where should I go in Admin Centre, what do I need to look at and change?

Any help very gratefully appreciated.