I power off if i wont be use

It's a privacy feature. You can disable if you want

Bazzite is install and use. Perfect out the box experience

First of all, I apologize for not answering your specific question about themes but.

To be honest, instead of struggling with themes on XFCE or MATE, you should consider that there isn't much difference in performance right now between those and Cinnamon or KDE Plasma. Both have a workflow very similar to Windows 10 out of the box, which would make the transition much more painless for your mom.

I highly recommend Linux Mint (Cinnamon Edition); it's the gold standard for 'it just works' and feels like Windows 10 without needing extra themes. If you prefer Plasma, Kubuntu or Tuxedo OS are excellent, stable, and beginner-friendly options that run great on older hardware without being 'bleeding edge' or unstable.

you can unload the tab with sideberry and with others extensions. If you just close the tab to save resources instead, simply unload it so you can quickly return to it and pick up where you left off.

They don't support Linux applications, they don't support the main Linux browser. They don't want you to use Linux

For self-hosted https://bitmagnet.io/is a very good and new alternative, has DHT crawler, and has an active development