Need to allow overrides on default website, but that (and all other options) greyed out

My main website has an .htaccess in its default directory that makes possible some of the tricks PHP does to keep the URLs looking nice (among other things). My understanding is that in order for the web server to stop ignoring this .htaccess, I need to check "Allow Overrides" in the site's preferences. However, that (and all other options) are greyed out.

I'm sure I'm missing something simple here, but if I simply create a new site with the same domain, it ignores it and sends all traffic to the default site. What can I do to either make those options editable or remove the default site?