HI Gar,
Here is how I use themes.
In the directory leo-editor/leo/themes are a number of theme files. These can be loaded using the File, Open Outline menu option in Leo. Opening an outline automatically loads it. Keep opening outlines until you find one you like.
Then close all of the theme files.
Copy the theme file you chose from leo-editor/leo/themes to ~/.leo/themes (you will have to create the themes folder here) and rename it something like this: Original file is BreezeDarkTheme.leo rename to BreezeDarkThemeLocal.leo
Now open your myLeoSettings.leo file.
Add the following to the headline of a new node as a child of your @settings node:
@string theme-name = BreezeDarkThemeLocal
Now save the file and restart Leo.
You should now be using the theme of your choice. From here you need to open the local theme file with Leo. The font sizing and colors are under Theme Settings in the tree.
HTH,
Chris