Tweaking your template:
I don't care if you tweak your template, but please note that unexpected and ugly bloggy consequences may ensue. Sorry, can't help you there.
Also, please beware:
If you have done any tweaks or changes to the code in your free trial template after you installed it at Blogger, those changes will be LOST if and when you reinstall the new template we send you if you buy graphics. We are not responsible for any lost modifications you have made to your template. Modifications do not include adding widgets, because you can restore any widgets you previously added once you install the new template.
Tweaking the graphics:
Some artists don't like it if you tweak their graphics. But if you bought the graphics and you want to include a logo in your header, for example, I don't have a problem with that (especially since I'd have to charge more to do it). So tweak away, but I'm not responsible for any unexpected results. Remember, BACK UP your original files. Back them up a second time for safe keeping.
What I do have a problem with (and all other artists and creative professionals agree) is redistributing the graphics, either for free or to make money. Just keep them to use for yourself and everything will be cool.