Other than changing the font size, you can easily change the default font style and change system font on HTC 10 with other fonts available in the Themes app. To apply a new font to your HTC 10, do as follows:
1. Open the Settings app
2. Scroll down to the Personal settings, and tap to open the Personalize settings
3. Scroll down to the Themes settings, and hit the Edit Current Theme option
4. Now, scroll all the way down to the Font and touch to open it
5. There you should see the entire list of available fonts for your HTC 10 (the fonts you have already downloaded)
6. Simply open a font and Apply it your entire device (You need an HTC account)
Note that, to download new fonts for HTC 10, you need to open the Themes app from app drawer, hit the 3-small-line icon on the top left, and go to the Fonts under the Store items.