There are at least three ways to change one button colour in WordPress: pick it in a theme setting, write a rule into the Customiser’s Additional CSS panel, or write it into your child theme’s stylesheet. All three change the screen identically. What differs is where the decision is stored, and that sets the cost six months from now.
Two kinds of storage
Theme settings and Additional CSS are stored in the database. A child theme stylesheet is stored as a file. The practical consequences of that difference are larger than they look.
The real damage comes from treating the Additional CSS panel as a scratchpad. It starts as two rules and, one urgent fix at a time, becomes two hundred lines. Those lines exist in no file, so anyone joining later can search the stylesheets forever without finding the cause. The screen and the code disagree, and nobody edits confidently in that state.
The test: where will the next person look?
Splitting the three storage options this way settles most cases.
Where the theme provides a dedicated setting, use the setting. Values a theme references in several places — the logo, the base palette — will always have a spot you miss if you override them in CSS instead. Usually somewhere unglamorous, like a mail template or an editor preview.
Conversely, every design decision without a setting belongs in the child stylesheet: your spacing scale, heading size steps, card corner radii. Those values need to sit together in one file so that a rebrand later is a single place to edit.
If it has already piled up
Moving an existing pile is straightforward. Copy the whole thing to the end of your child stylesheet, compare the pages, then empty the original panel. Move it in one go and check immediately — leaving half behind gives you the same rules in two places, and later nobody can tell which one is winning.
There is a side benefit. Reading two hundred lines on one screen exposes the places you wrote the same value differently. That observation is the starting point for design tokens, which a later series covers.
Articles on structural tidying live in the Themes & plugins archive, and if you want the cleanup and the upgrade handled together, that is what our optimization program covers.
Next part
Some changes CSS cannot make, because the markup itself has to change. That is what template overrides are for — and part four covers both the rules and the price.