You have a few validation errors in your markup that are going to cause some issues. One that I noticed is you have defined a div with an id of "content" more than once, once around here
Code: Select all
and again further down in your code. I would consider changing that first content declaration to be named something else, as it appears the second one is the actual declaration from the default theme that you are modifying.
http://css-tricks.com/the-difference-be ... and-class/
has a great explanation of the difference between id's and classes, and when to use each. If you're going to be taking this on yourself, I would definitely check out some other articles on that site as its a great knowledge base to further your CSS know-how
There were several other errors of the same type. Visiting your site, right clicking, selecting View Source, and copying and pasting all of it into http://validator.w3.org/
"Validate by Direct Input" tab will show you all of the errors. Don't get too shocked, the default OpenCart theme ships with a lot errors that look like "Line 197, Column 114: general entity "product_id" not defined and no default entity", which aren't going to effect the layout of your site and which I ignore. But use your browsers find feature to find "already defined" on the list of errors it gives you and you will see the id declarations that you've made more than once.