Board Thread:Support Requests - Designing Your Wiki/@comment-44745594-20200927035923/@comment-45117243-20200927135014

Update: Those CSS definitions seems to give a partial solution:

background-size: auto 800px; background-attachment: fixed; background-position: center;

However, for very narrow screens (like desktop view on a mobile phone) 800px is probably too wide. You can handle those with @media rules.