Presently there is no way to change the default font style of all the components in app designer. But I have passed this issue to the teams concerned, they might consider this for future releases.
Regarding the query of changing the font style of tab group component. I was unable to find the user interface to change the font style.
But I was able to accomplish this by changing the font style from the code view.
For example, I added the line
app.TabGroup.FontName='Roboto Black'
In the startup function of the app to change the font style to “Roboto black”. (Line 17 in screenshot attached below)
Then I was successfully able to convert font style to Roboto black after running the code.
Hope this helps!