How can I change my font sizes without using a theme?

You may use the Formatting toolbar to change the font or color of objects on the Diagram Window. The toolbar lists all the fonts that are available on your workstation. It also has separate buttons to modify the color of the text, object, and outline color. One of the benefits of this toolbar is that you can use the options to quickly apply formatting to individual attributes without having to add another theme or alter an existing theme.

Please try the following:
Open your model
Press CTRL-A to highlight all the entities
The Formatting toolbar should now appear at the top
You can now apply the font and sizes to the objects this way.

Figure 1.  Formatting toolbar font change

 Figure 2.  Formatting toolbar font size change

 Figure 3.  Changed font size to a larger size