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Apologies for all of the questions :littlecfrown: Every time I think something's going well, I manage to mess something up again.
After I'd finished editing my css (Thanks again for helping me figure it out!), I logged out of my admin account (front end) and tried to look at the main page. Now, any time I try to view a page, I get this:
http://gyazo.com/58a63265a157629727b...png?1335208268

I thought the problem might be that Main was capitalized, since in the file manager/files/filezilla, Main isn't capitalized, but fixing that didn't help at all. I changed the CHMOD of functions.php and template.html to 777, but that didn't work either. It was working fine before I started editing the css, so I can only assume that I must have accidentally messed up the functions.php file somehow around line 459.
The ACP still shows up fine, I just can't access it because I can't log in from the front end of the site. I can still edit files through the file manager, though.
Thanks again for answering all of my questions ^^

Well look at table prefix.users_options, what is the value for field 'theme'? If the theme is 'Main', change to 'main' and it should work for you. I have no idea why you are having this capitalized theme name issue though.

Fixed, thank you! It shows up fine now ^^

You are very welcome, glad it fixed your problem.