Hi Manfredk,
Wordfence is a plugin that scans for coding pattern and functions that could be used by hackers.
The PHP function eval http://php.net/manual/en/function.eval.php as well as base64_decode http://php.net/manual/en/function.base64-decode.php are all built in PHP core functions.
The Wordfence plugin does not have artificial intelligence, it is not able to differentiate if those functions are put to good use or bad use. It can only show you warning when it detects them, so that you can get a programmer to look at it, to determine if it is malicious code.
In this case, the Author of Evolve Theme, has step out and say that it is safe, and it has also passed WordPress checks, and the Theme is still available for downloading. So we can say that it is safe to use.
Thanks!