What version of FMP are you using? There are several packages that are specifically for FMP "interface" elements. But depending on the version, they may or may not work for you.
you can find PNG and SVG icons/graphics for web pages that work very well in FMP. A lot of them are free.
I'd use standard Filemaker buttons instead. Using graphics is kind of "old fashioned".
If you want to use lightweight graphics and very nice layout themes you may want to take a look at this:
Hope you like it.
absolutely, FileMaker buttons (depends of version which supports them?)!