Currently there is not a solution and it's not possible to change directly the theme inside the app. There is a third-party app that let's you change the text pointer for all the apps. It's not an official software so we can't recommend to install it, we speak about it here only for completeness of information.
You can learn more by following this link:
And reading the post about Mousecape by user alpriest, commented on Nov 4, 2016.