Theme development

This may be a dumb question that seems basic, but I have been unable to find anything online that will answer it. Is a Drupal 9 theme intrinsically compatible with Drupal 10, as is without any upgrading? Is a Drupal 8 theme?

If not, are there simple standard mods that I can do without hiring a theme developer that will make it work?

Hello,
I need a very simple forum function for my site and I am currently trying out the forum module in Drupal Core. My Drupal is version 10.1.8.

But now there are the buttons "preview" and "add another item" under the comment function and to the right of the comment function there is also a sort function. In the code itself, this appears as a tabledrag function, but modules such as draggable views or weight are not built in.

Can I hide these functions somehow, because I don't actually need them and the function is just a nuisance?

I have a website with the default Olivero theme. I want when the navigation bar display switches to mobile mode, the menus are automatically expanded. Like in this screenshot :

https://ibb.co/zXbjYLB

I created a scripts.js file in my Olivero subtheme. I imagine I need to add some code, but no idea what to put.

I have added some custom fields to a content type, and when I put the site into twig development mode I get this from a dump:

Good morning, after days I find myself forced to write for help... I have already tried many installations of bootstrap5 on the drupal10 platform. As for the CSS, I can make changes (inserting variables in _variable_bootstrap.scss and recompiling) while I can't see the bootstrap views (accordion, carousel, etc.) working. I believe the problem is related to the javascript code that is not considered. What do I have to do though ?
Thank you

Recently, I developed a subtheme using Boostrap and then I deployed it to production. When I was developing the theme locally I had no errors, but as soon as I deployed it to production, I started getting these two error messages:

GET 404 ERROR: https://website.com/themes/mytheme/css/theme/style.css?sa5rad

The resource from “https://website.com/themes/mytheme/css/theme/style.css?sa5rad” was blocked due to MIME type (“text/html”) mismatch (X-Content-Type-Options: nosniff).