All elements can be given a border, which have color, thickness, outline and join styles.
An element's border can be given both a color and an initial width.
You can also add an extra border ring, with an alternate color. This alternate color generally doesn't match the primary color. Additional outlines will have the same width, though each will be narrower than the primary outline.
The join style, though subtle, can provide a pleasing accent when paired thoughtfully with your comic. Use squared edges for a rougher look, rounded corners for a smoother feel, and mitered corners for a more geometric look.