The Strange Readme Button in mycuisine

For some reason the readme button is split into two pieces and I can’t seem find the reason.

Here is the effect:
https://i.imgur.com/numds.jpg (close up)
https://i.imgur.com/Xwgan.jpg (normal)
The website is https://www.libaneeskoken.nl

I reset the styles file with no effect. The only change I have added to the styles.css removing the searchform but even resetting this has no effect.
#searchform { display: none; }

Would anybody know where this problem comes from?

Thank you