.mw-infobox is in use solely by [[ https://codesearch.wmflabs.org/search/?q=Html%3A%3AinfoBox&i=nope&files=&repos= | Html::infoBox ]] by Web-Installer.
It shouldn't receive different color appearance other than one of the standard boxes .warningbox or .messagebox as they are also available in 'shared.css' and are aligned to Design Style Guide.
This has last been touched ~8 years ago.
- Remove all extra visual properties aside of pure box layout properties
- Replace outdated PNG icon with current standard icon and consider SVG
- Replace infobox usages and extend successbox, warningbox and errorbox – https://gerrit.wikimedia.org/r/#/c/mediawiki/core/+/537245/
- Remove internal HTML::infobox method