Appearance -> Customize -> Additional CSS
Here, copy and paste this code in the space:
.widget-title { font-size: 24.5px; }
You can replace the font size with any number you like.
]]>h3.widget-title { font-size: 24.5px; }
I did try to use this code in the Additional CSS section and it seems you’re right about it not working there, so I’m a going to look into this problem a little more.
I did however find an article that might be helpful to check out in the mean time:
Add Custom Styles to WordPress Widgets