On Vivaldi Themes, click on “Download theme” to preview the theme before installing it.
Then you have 1 minute to see how the theme looks. Make sure to check the Start Page to see how the awesome background image looks behind your Speed Dials. Other places where the user interface colours are clearly visible are Panels, Settings, Mail, etc.
When you’re satisficed with the theme, click “Install” in the pop-up to add the theme to your Theme Library. If it’s not really what you were looking for, click “Cancel” or let the preview time run out.