Hey Richard,

The base theme for 2.5 is still Standard Totara, so custom themes created for 2.4 based off of the Standard Totara theme should still work in 2.5. Custom themes not based off of the Standard Totara theme might have a few area's in need of an update, I would reccomend checking the element library (under site administration > apperance > theme > element library) to make sure everything is still looking right after the upgrade. We also added a new theme called "Standard Totara Responsive" which is based on the Moodle 2.5 Bootstrap theme, however that is still somewhat experimental at this point.

Cheers,
David