Your theme defines “Ubuntu, sans serif” as default font. On your computer you might have that font, on your mobile not.
Solution: add this to your header (between
</head>, before the stylesheets):
<link href="https://fonts.googleapis.com/css?family=Ubuntu:400,400i,700,700i&display=swap" rel="stylesheet">
You could do it properly with
@import rules in your stylesheets, but the above will be a quicker fix.
Edit: On closer look there are several fonts missing from that package. See here: