In both the desktop and desktop browser apps, you can increase the font size by zooming in, just as you would with any website in your browser. To zoom in on the desktop app, you would use Ctrl+Shift+Plus and to zoom out, Ctrl+Minus.
In Settings you can also change the font family. Please note, the font settings are "global" and affect the fonts wherever you type. You cannot however select one block of text and change the font.
You can select any text and make it bold, italic or underlined. Simply highlight the text, and use the following keyboard commands:
For bold, use the CTRL+B keys
For italic, use the CTRL+I keys
For underline, use the CTRL+U keys
You can also change the "Theme", which will provide a variation of the look and feel of the font and background colors.
With the Pro membership, you can access more fonts and themes... but please note, the additional themes will only be visible if you are using the desktop or desktop browser apps.
On the mobile apps we currently have the default theme for all users... as well as the dark theme for Pro users.