Question 1: How can i make the top bar (adress bar, tabs, book marks etc.) dark without making site dark?
Question 2: Which setting should I pick in settings > languages(about:preferences#languages) > website languages for privacy?
Question 3: Am I fine if I deactivate DNS over HTTPS because I use my ISP resolver and setting to standard will/might use a different one
On question 3: yes, that’s a reasonable choice. With DNS over HTTPS off, Firefox uses whatever resolver your system got from the router, so your ISP’s. The default setting is the one that may switch you to a different provider (in some regions it enables DoH with Cloudflare or another partner automatically), so setting it to Off is how you make sure it stays with the ISP.
The trade-off is that plain DNS goes unencrypted, so anyone on the path can read the names you look up. Your ISP sees them either way since it resolves them, and it also sees which IPs you connect to, so for the ISP itself you are not giving up much.
Are u sure that the dns will go in plain text?