how to change from dark to light background

You might have something else in mind, but for content the setting can be to follow system or an explicit color scheme: Change website appearance settings in Firefox

No........... I want the background that displays all my emails to be lighter....or in Light mode... right now everything is too dark.

No........... I want the background that displays all my emails to be lighter....or in Light mode... right now everything is too dark.

If your browser preference is set to Light already as linked above, and a site shows you dark theme, check with the site operator's help pages how to configure their app to use a different color scheme.