Fix minor regression of #13250
plugins/Morpheus/stylesheets/theme.less got deleted, but only the include in
plugins/Morpheus/stylesheets/main.less has been removed.
Because of this less keeps a
<a class='mention' href='https://github.com/import'>@import</a> "theme"; in the CSS file and the browser tries to fetch
https://demo.matomo.org/theme which causes a 404.