Not sure what do you mean, but I clearly see you have the code in
default.css like you've mentioned. And your current website color is definitely white.
What you experience could be a result of CSS file
caching on your server. On the server where we keep miniBB, I experience this, too. If you update the CSS file, you won't see immediate results until you fully clear the browser's cache, or check it in another browser, which previously didn't open your website, i.e. it will have no cache regarding it.
I'm not really sure where it could be changed in server configuration (and if it's possible at all on the non-root level), but the quite easy method to see the fresh CSS is adding come random string to it, or reloading CSS with hard-refresh pointing browser's URL to it directly.
E.g.:
1) in
templates/main_header.html by default you have this:
<link href="{$main_url}/css/{$skin}.css" type="text/css" rel="stylesheet">
add some random number (f.e. current date):
<link href="{$main_url}/css/{$skin}.css?20230724" type="text/css" rel="stylesheet">
And each time you update the CSS, add new random number.
This will work in all browsers, and later after some time when the cache re-initiates, you can remove this number, leaving the original code.
2) Your miniBB CSS is located by default under
/css/default.css. For example, on miniBB forum it's located here:
https://www.minibb.com/foru
ms/css/default.css
It's also found in HTML source, and in most cases is clickable.
So, open this file in your browser directly, and RELOAD it, clicking the proper Button or Icon. You should see an updated code, and pointing the browser back to your forum or website, RELOAD it, too. Then you should see the updated results.
Inform me if it helped :-)