Font all messed up after changing theme

Hi,
This morning I purchased a new theme and upon activating it, I see that all the font in my previous blog posts have been messed up. Different sizes and types are now present in each post. Beforehand, everyhing was in order. I never pasted anything from elsewhere into my blog posts without removing the formatting. So I’m not sure why it should be different.

I’m no expert on CSS and the like, but I suppose that has something to do with it. I would be very grateful if someone could tell me how I can fix this issue and have all my blog posts with the same font. Where in wordpress do I control the font?

Here is my website
I have my old theme activated right now, but am anxious to move to my new one asap.

Many thaks for any help,