osTicket Core, v1.10.4 supported responsive theme released at https://www.osticketresponsivetheme.com/osticket-theme-ostone
We never supported custom themes with core osTicket. People modded osTicket and created their own themes, yes, but these are not nor were they ever officially supported. As @ntozier said, themes are on the roadmap for the v2.x
series so please stay tuned.
Cheers.