Appearance → Customize → Style → Accent color. Default #0000ee — the browser’s own link blue.
Why the default is browser blue
Because that is what style.css sets links to, and the theme would rather open the picker on the colour the page is actually using than on an empty swatch you have to guess at. It is also, for what it is worth, the most recognisable link colour there is.
Clearing the picker returns the accent to that same value, like every other colour in the section.
Where it is used
- Links in post and page content
- The Primary header button
- Interactive accents through the theme’s own components — focus rings, the current item in a paginated listing, form controls
It is deliberately a short list. An accent used everywhere stops being an accent.
Picking one
Take it from your own material. If you have a logo, the accent is in it. If you run photography, pull it from a typical picture. A colour that already appears on the site looks chosen; one from a palette generator often does not.
Mid-dark works best. The accent has to hold white text on a filled button and read as a link colour on the page background. A pale pastel fails the first; a near-black fails the second and just looks like text.
Check the contrast. Against white, a link colour needs 4.5:1 for WCAG AA at body size, and a filled button needs the same ratio between the fill and its label. Any online checker answers in a second. The default #0000ee is about 8.6:1 on white, which is why underlined blue has survived thirty years.
News sites go dark. Navy, oxblood, forest — a saturated mid-tone at high contrast. A bright orange or cyan that looks good on a product site reads as a promotion on a masthead.
On a dark background
If you have built a dark site, the accent has to be re-picked: #0000ee on #111111 is about 2:1 and effectively unreadable. Move to a light tint of the same hue — #7aa2ff rather than #0000ee — and check it against the background you actually set.
Colour is never the only signal
Links in Stoat’s content are underlined as well as coloured, so a reader who cannot distinguish your accent from the body text still knows what is clickable. If you remove that underline in Additional CSS, put something else back in its place.