Hi @juliane,

Welcome to the Community! 

And I would also like to regrettably welcome you to the world of Outlook email formatting 

I had this same discussion with another Klaviyo user in this thread:

TL;DR: This is an issue on the Outlook end, not with you or with Klaviyo. Regardless of which platform you choose to send email campaigns from, there are known compatibility issues within this inbox provider which cause display issues for some design elements. Because of this, an email may look slightly different in Outlook than when you initially design and preview in Klaviyo.

The fix: To remedy your specific use case, I might recommend uploading a graphic (i.e. image) for your header with the background color native to the graphic itself to prevent it from being changed across various inbox providers. As Outlook cannot change the colors of images themselves, this will result in  a more consistent experience for your subscribers, regardless of the device or inbox provider they are using.  

You can use Photoshop or a tool like Canva to build a header image with your desired background color, and import that graphic as an Image block to the Klaviyo template editor. 

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Hope this helps! 

-- Ashley Ismailovski