Why is the theme for my messaging not changing?

[ConfusedandDesperate]

I have the galaxy s6 and I use the messaging+ app. When I first got my phone I decided I wanted to customize my messaging instead of having it look like the theme I applied. I've found a theme that I really like and I want it to be applied to my messaging as well. However, it's not changing. It's staying the way that I customized it. I cleared all the data, restarted the app and still nothing. Does anyone know how to make my messaging change with the theme as well, like it's supposed to? Thanks in advance.