Thanks for your postsuggestion.

I have added this request to the tracker:
Issue 2452 – Use different color for active tab in dark theme
You can vote for it there.

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Alternatively, you can enable hi-contrast mode:
https://winscp.net/forum/viewtopic.php?t=33807#120447

In dark mode, the active tabs are almost the same as the inactive tabs, which often leads to misperceptions. Suggestion: Please make the active tabs a different color.