If a theme doesn’t support custom styling out of the box, and thus requires modification of the original CSS files, this can easily be retrofitted by making use of CSS’ @import rule:

Simply create a new file in the theme’s folder^[1] (e.g. wp-content/themes/[name-of-theme]/custom.css) and add your CSS code^[2](the file should be available from the WordPress admin panel’s Theme Editor).
Then add the following line to the top of the theme’s main style sheet:

@import "custom.css"

Later, when upgrading the theme — after making sure that custom.css is not deleted — all that’s required is to re-insert the @import code.

1. Initially, I wanted to create a central custom.css in the themes parent folder — unfortunately, that file wouldn’t show up in the Theme Editor.
2. Don’t forget about CSS specificity; from my experiences, this is often the culprit when a modification doesn’t seem to take effect.