According to my understanding, the CSS looks indeed invalid but it hasn't been changed recently, this is the code: https://github.com/xwiki/xwiki-platform/blob/b167253b4b385c2b8bebeaec79f25f55bf2c201c/xwiki-platform-core/xwiki-platform-notifications/xwiki-platform-notifications-ui/src/main/resources/XWiki/Notifications/Code/NotificationsDisplayerUIX.xml#L857

The missing color "buttonSecondaryGradientColor" is defined at https://github.com/xwiki/xwiki-platform/blob/9298126038418d2364d7164783774662db2ef463/xwiki-platform-core/xwiki-platform-flamingo/xwiki-platform-flamingo-skin/xwiki-platform-flamingo-skin-resources/src/main/resources/flamingo/colorThemeInit.vm#L64 but might be removed here: https://github.com/xwiki/xwiki-platform/blob/9298126038418d2364d7164783774662db2ef463/xwiki-platform-core/xwiki-platform-flamingo/xwiki-platform-flamingo-skin/xwiki-platform-flamingo-skin-resources/src/main/resources/flamingo/colorThemeInit.vm#L104 - I also couldn't find any further references to this color and I suspect it just doesn't exist anymore.

I wonder if this could be cause by an update of the LESS parser that now understands this CSS while it might have just treated it as an opaque string before.