I want to share with you this awesome tip that i can found in the miui forum (thanks to sianman)
If you don't like the fonts in the miui theme store, you can add more with the iFont app (you can find it on PlayStore)
Follow the step:
1. Launch the iFont app.
2. Find and tap the font you would like to install.
3. Tap Download.
4. Tap Set.
5. On the Setting font prompt, tap OK.
The amazing feature it's that than you have tap OK immediately the MIUI Themes app start automatically, and you need to go on Category > Component > Font and select the font already download with iFont.
Tap Apply and reload the phone to see the changes.