Changing the colours and fonts
The palette and fonts are defined once at the top of styles.css. Change them there rather than hunting through the pages.
Every template defines its palette and fonts once, at the top of styles.css, as custom properties. Change them there and the whole site follows.
:root{
--ink:#14161a;
--paper:#f7f4ee;
--accent:#c0311c;
}You do not need to edit the individual pages. Nothing hardcodes a colour further down.
If you change the fonts
Templates that use Google Fonts load them with a <link> in the <head> of every page. If you swap the font, update that link on all of them, or one page will silently fall back to a system font.
Check the contrast afterwards
Accent colours in these templates were picked to stay readable against their background. A new colour can look right and still be unreadable for someone with low vision. Check text against its background before you launch, especially small text and anything on a coloured button.
Still stuck? Email [email protected] and say which template and what you expected to happen.