Changing the background of a web preview tile can be done in the Theme Editor. If you're using the same web preview tile background on your site, you can use the web preview tile under the Grid Sections editor or a Style Preset if you are using different web preview tile backgrounds in one site.
Using the Web preview tile editor
Go to your site's Theme
Go to the left pane, look for Grid Sections and expand
Look for Web preview tile
Change the background by entering the Hex color in the box or clicking the color and selecting it from the color picker.
Using a Style Preset
Go to your site's Theme
Go to the left pane, look for Style Preset and click the +New sign to add a new preset
Enter the name of your new preset
Expand Web preview tiles
Change the background by entering the Hex color in the box or clicking the color and selecting it from the color picker.
Note: Changes to the theme apply to all sites that use the same theme. If you don't want your theme changes to affect other sites, it will be better to add the Style Preset or duplicate the Style Preset that you prefer before making any changes to the theme.