Good job spotting that markup issue @zeniph.
@gouldimal Those PHP files are exactly what you want. WordPress uses the PHP language for theming. The files are a mix of everyday HTML tags and PHP tags that are replaced with the dynamic content from the software. Don’t be intimidated by the PHP tags, it’s actually pretty simple to understand once you tinker a bit. I suggest starting here to learn more about templates: https://codex.www.ads-software.com/Stepping_Into_Templates.
Regarding your theme, you might check that any <div>
s being opened in header.php are being closed in footer.php. These two files are used as the top of each page and the bottom of each page, respectively, and it’s easy to drop a closing or opening div in one of them and not realize it.