See for example: https://en.m.wikipedia.org/wiki/Special:MobileDiff/837139723
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Ok, but exactly which file do I need to modify or which file contains the css of this button.
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https://gerrit.wikimedia.org/g/mediawiki/core/+/master/resources/src/mediawiki.ui/components/buttons.less includes mw-ui-button and mw-ui-progressive class.
FYI, you can use codesearch to find code.
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@Psuranas: Sorry, this just got fixed by someone else earlier this week (but in a duplicate bug).