One of the big questions that float around often on the forums is "what is and is not ok to edit in regards to style?"
I will attempt, even though It's really tough to answer because the CSE Terms of Service (TOS) are somewhat broad and confusing (stating you can't modify attributes1, while giving you tools to modify some of them), but I think I've found a solution!
I would think it's safe to assume that Google wouldn't feature anyone violating their TOS. So you can take a look at what the people they're featuring are doing to serve as a rough guideline.
Of course, to be absolutely sure, you should contact Google. For further assistance, the Google Custom Search group is an excellent resource as well, except, it's users helping users, so if you've got a TOS related question that really seems iffy and you're not sure, I would still contact Google. However, unless you subscribe to the business edition (currently set at $100/yr), they may offer you no support. Google CSE is currently still in it's Beta version, so I would be on the lookout for an array of new improvements in the future.
Sources: 11.4 Appropriate Conduct. You shall not, and shall not allow any third party to: ... (j) modify, adapt, translate, prepare derivative works from, decompile, reverse engineer, disassemble or otherwise attempt to derive source code from any Service or any other Google technology ...
Excerpt from CSE TOS http://www.google.com/coop/docs/cse/tos.html