Gmail – Is it possible to change the background color of all incoming emails in Gmail

I'm not sure that you can do this directly in Gmail. However, you could either selectively forward emails to an alternative account OR download your Gmail using POP3 to the Mozilla Thunderbird email client (which is available under Linux).

If you are filtering more than a handful of emails it would be easier to download your emails en masse using POP3 and use the filtering capabilities of Thunderbird.

Thunderbird supports many add-ons. The ImportExportTools add-on for instance appears to do what you are after, allowing the export of emails to single or multiple text files (there are no doubt others).

Thanks to @Tschareck for clarifying that Gmail filters do not allow you to forward emails already received.

You'll need to select a theme that changes hourly or at least based on the time of day.

In Gmail Themes there are four types of backgrounds you can choose, as indicated by the icon in the lower right of the background preview image:

• None - Static and one image only
• Sun - Changes based on the weather, set by your location setting
• Clock - Based on time of day
• Strip - Depends on the day of the week

That's the closest you'll get for now until they allow you to upload or select an album to flip through as a background wallpaper on X time increments.