Generally you'll just add the outlines property to whichever style you need it for! This template does mean that doing something like gui.dialogue_outlines = ... will not work because the whole point is to avoid all those implicit gui-style properties. But if you want outlines for the dialogue text, you can add it to style say_dialogue, or for all text, to style default. In general, if you want to add a property to something, hover over it with your mouse and hit shift+i to find the style you need to add that property to.
Follow the examples in the docs: https://www.renpy.org/doc/html/style_properties.html#style-property-outlines
The standard is outlines [(1, "#000")] for example, which will be one outline, 1px wide, that's black.