It looks like you have the Search banner theme component installed. To change the headline and subhead text navigate to:
Hamburger Icon (top right) -> Settings -> Customize -> Components -> discourse-search-banner
Below the theme settings there should be two input fields underneath ‘Theme Translations’, which you can enter both the heading and subheading there.
If you want to change it only for your specific user:
Navigate to:
- Profile Icon (top right)
- User icon
- Preferences
- Interface
- Switch Theme to ‘Light’
If you want to change it for all users:
- Hamburger Icon
- Admin Settings
- Customize
- Change theme from Light to Dark (select Theme is enabled by default)
Note, you may have automatic dark mode switching enabled, in which it will depend on your computer’s selected theme.