Child Themes « WordPress Codex

Child Themes « WordPress Codex: "The part after the closing */ of the header works as a regular stylesheet file. It is where you put the styling rules you want WordPress to apply.

Note that a child theme’s stylesheet replaces the stylesheet of the parent completely. (The parent’s stylesheet is not loaded at all by WordPress.) So, if you simply want to modify a few small things in the styling and layout of the parent —rather than make something new from scratch— you have to import explicitly the stylesheet of the parent, and then add your modifications. The following example shows how to use the @import rule to do that."

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