Unfortunately CSS is a tricky technology when it comes to browsers. They all tend to rendered their own way, and the best solution is to try to keep it as simple as possible.
That said, you can mix percent with fixed values whenever you need.
For example, images don't need sizes to be set, since they already have one. So, the page renders the image in its original size (unless you want to change it with CSS, but I would change the file itself for better image quality).
Many of the templates break in I.E or Firefox, especially if you try to move things around the page with the layout configuration options in step 5.
I believe there was no time to really test the themes.
It is very frustrating when developing, so I stick to just 2 browser: IE and Firefox, and don't worry about other browser because it becomes unmanageable. There isn't enough time.
I usually use 100% widths and 0 for padding and margin for elements that span the whole page.
Inside those I try to use a mix of fixed for the sidebars and auto or 100% for the content (whatever prevents the page from breaking).
Depending on the HTML generated by the software, you may also need to mix relative and absolute positions to achieve your goals.
Especially if you want it to be displayed almost exactly in at least the two mentioned browsers.
Good luck, and don't give up...