The Google AMP team has increased the AMP custom CSS limit from 50KB to 75KB, that is a 50% increase. This should be fully live by the end of February said Weston Ruter, a Developer Programs Engineer at Google on Twitter:

Huge hat tip for finding this goes to @PeterNikolow.

Weston from Google added “It should be live by the end of February. (We’ll make sure to release an update to the WordPress AMP plugin as soon as this happens.)”

Forum discussion at Twitter.