Hello Martin,
I have tested it and the custom css file is loaded. Firstly, ensure you've actually made the changes to the file. Check the file again, in case permissions are causing some issues.
Secondly, ensure you are clearing your browser cache, and Joomla cache (Admin > Site > Clean Cache), as some changes will not even show up until you clear your cache.
Third, your code is wrong. The html code is:
<body id="rt-variation" class="font-family-helvetica font-size-is-default menu-type-fusionmenu col12 ">
You can see that the body both using id and class. So to override it, the css should be:
#rt-variation.font-family-helvetica {font-family: tahoma;}
Notice that there is no "space" between "#rt-variation" and ".font-family-helvetica".
Just change the tahoma with your font you like.
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