Go to Colors and update the Primary or Accent swatch. Every component reads from these values, so the change applies sitewide immediately after saving.

Go to Branding → Logos and choose an image for Primary Logo. Dark/Light variants are optional, for headers with a transparent or dark background.

Settings save immediately via the Save Changes button in each section. If a change still isn't visible, clear any page caching plugin and your browser cache.

Use Reset This Page on any settings page to revert that page to its defaults, or Reset All Settings under Import / Export to start over completely.