The CSS for attractive buttons is duplicated in multiple extensions, with the latest and greatest in extensions/Agora.
In order to implement improved login and create account forms in core (bug 44628, gerrit 30637), this CSS needs to move into core. E.g. resources/mediawiki.ui/mediawiki.ui.default.css plus a vector version.
(Nothing currently depends on the module 'ext.agora.base' implemented by extension Agora, again it's all copy and paste.)