Appearance and Theme Color are separate settings: one picks light or dark, the other picks the accent you see on buttons and headers.
Why this matters
Appearance and Theme Color are separate settings, which is easy to miss: one is light or dark, the other is the accent colour used on buttons and headers.
Steps
Open Settings, then Appearance.
Light
Dark
Midnight Theme
Back in Settings, open Theme Color to change the accent.
What happens next
The change applies everywhere in the app at once. Nothing you have saved is affected.
Good to know
Choose from unlimited color options
Unlock custom theme colors and premium features
In dark mode the accent is fixed, so a custom colour only shows in light mode.