GTK CSS has a selector "pseudoclass" for [state active for tabs](https://docs.gtk.org/gtk3/css-overview.html#theme-labels-in-the-first-notebo...), but the `geany-*` settings for text are state independent, they depend on Geany internal state (file changed, readonly etc) which are not available to GTK and Geany ignores state active.
But you probably can do it for the rest of the tab that Geany does not control using normal GTK, even if you can't control the text colour by state, a flashing purple and orange striped background should make the tab visible ;-)