I just saw there was an upgrade to version 3.5.0, so I gave it a try. Now my website doesn't have a proper working theme anymore. The fields show up and in the right order, so I guess Silverstripe has access to the database and uses the SS files I made, but nothing is formatted. It's as if I didn't have a CSS file anymore. I think with the upgrade, my Page.php file's controller "Requirements" lines giving the CSS files got broken. Did "ThemeDir()" change?
Thank you for your help,

Despite trying a "dev/build" and also "?flush=1", nothing changed. I finally managed to fix it up, though. As I posted before, I suspected the main controller giving the requirements lines to be broken, so I opened the Page.php file, just switched the order of two of my requirements lines to fake a change in the file and saved it, so next time I ran "?flush" Silverstripe saw a change in the file and reloaded it. Now everything's fine (as far as I know).

Here are some updates. As I was working on my new website I just realized my extension to SiteConfig I made last week doesn't work anymore. I had made one just like in the tutorial which would give me an extra tab called "Social" that would contain all my social networks links. It used to work last week, but since the upgrade the tab I made doesn't appear anymore. Anybody knows why?

I just discovered the problem. After the upgrade, it seems the file config.yml gets overwritten by the original version of it, so the line that specifies the custom theme changes back to the original theme and my last line that specified my extension to SiteConfig got erased. I also noticed these 2 new lines and wonder if I should keep them:

Debug:
  friendly_error_httpcode: true