You can specify a different Outline Color as well as a different Material to use for a font outline.
The size of the outline is specified in Slate units, but when the font's scale is 1.0, 1 Slate unit is equivalent to 1 pixel.
One interesting note is that you can specify whether or not to use Separate Fill Alpha.
When this is enabled, the outline is translucent
where the filled area will be, allowing you to adjust the alphas of the font and the font outline independently. When it is disabled, the font is overlaid
on the outline, so the alphas are additive and the outline color and Material is visible through a font with an alpha less than 1.
Font Color Alpha = 0.5, Separate Fill Alpha disabled
Font Color Alpha = 0.5, Separate Fill Alpha enable