Leo themes always use monospaced fonts for the body text. This is understandable because Leo is so often used for programming. But there seem to be many users who use it for non-programming writing activities. In these cases, monospaced fonts aren't always the best because their readability is not so good.
Trying to change Leo themes can be tricky. But you can change the body font easily, just to see if you like it another way. The font will be changed for all nodes in an outline. With this method, the new font will be used until either you change it again or reload the outline.
Create a node somewhere in your outline with the following contents - change the font name if you prefer another one:
editor = c.frame.body.wrapper.widget
editor.setStyleSheet('QTextBrowser {font-family: Verdana;}')
You may want to change the font size, too. Use this version instead, with the size you prefer:
editor = c.frame.body.wrapper.widget
editor.setStyleSheet('QTextBrowser {font-family: Verdanal;'
+ 'font-size: 10pt;}')
To activate the new font, select this new node and press CTRL-B.
Attached is a screen shot showing the effect. Notice that I have also changed my Leo theme to use a non-monospaced, UI-style font for most of the other visible parts of Leo. I find I like this effect, too.