New to natron as well and also not having the right font. But I think it’s an alpha problem. Make sure to set the alpha on the BG to 1. Check the alpha before write out. If you want a solid bg it should be white.
I think it’s related to this Should the Natron viewer display unpremultiplied RGB? - #15 by Shrinks99
If you want to learn more you should read about premultiplied an unpremultiplied alpha. It’s little confusing, but a very important thing to understand.