Currently the MediaWiki installer for 3rd parties is configured to use Vector. This is likely to break with future desktop refresh iterations. I suggest we use the Fallback skin for the installer rather than rely on this hack.
= Background
Let's not hardcode the skin module here to avoid risking the desktop refresh from breaking the rendering of the installer:
https://github.com/wikimedia/mediawiki/blob/73c20062e0d3e0dcd94d841baa7ee7e605a688c7/includes/installer/WebInstallerOutput.php#L154
The existing setup also double loads CSS.
Instead let's use the hooks system to determine what module should be used (if any).
In future when the skin system is in a better place we should be able to use any associated templates with the skin to render the associated HTML but that is out of scope for now.
Current appearance (using Vector's styles but own HTML):
{F31718586}
Appearance with Fallback skin:
{F31718588}
https://gerrit.wikimedia.org/r/#/c/mediawiki/core/+/581771/
Appearance with Fallback skin + installer styles:
{F31718603}
https://gerrit.wikimedia.org/r/#/c/mediawiki/core/+/585042 Installer specific styles