As stated in the title, are there any websites out there to play thematic games in Blitz (or quick time control) ? The idea is to practise a specific opening, starting from a precise position (arising after the exchange Grünfeld for example). I know a french website where it's possible but for corresponding games only. Thanks in advance.
Lichess has what you are looking for:
But you may have to check out when a specific opening theme may be scheduled in their tournament table. Also Lichess has a feature to start your own thematic tournament at any time of the day, but attracting players to the custom tournaments typically does not work.
Chess24.com used to do this - https://chess24.com/en/read/news/new-thematic-tournaments . I don't know if these tournaments are still running.