Its is difficult to find tab of current file if there are many files opened at once since all of them have same colour. The current tab should have different colour so that user can find it quickly among other tabs.
Sounds like you're using a bad GTK+ theme, it looks OK with the theme I'm using (Materia Dark), which makes the current tab a lighter colour and puts a blue bar under it. This isn't controlled by or specific to Geany though.
You can also customise your GTK theme by the usual methods, or Geany will read specific customisations from geany.css in its config directory.
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