Image galleries won’t work with my custom theme

I’m hoping someone else has had this problem and figured out a solution. It’s probably something stupid that I’m overlooking. Basically, I designed a custom theme, and now the image galleries appear to have all styling and scripting stripped from them. When I activate the default theme, the galleries work just fine. On my theme, the thumbnails are listed down the page with no style. And when you click them, it opens the image in a new tab/window. I have the nextgen gallery plug in installed, but even when deactivated the default wordpress image gallery does not work. The nextgen plugin and default image gallery do work when I activate the default theme. I’m using the most current version of wordpress and nextgen, but I don’t think it’s a nextgen problem. I’m positive it’s a problem with my theme. Where is the code/styling/etc. in the default theme that handles images? I must be missing something in my theme. Let me know what other info I can provide to help resolve this.
Thank you!