You can customize your event page's appearance by changing the promotional poster image, choosing to use a blurred version of that image as your background, or choose a solid color as your page's background.
On your event page, click the pencil icon in the upper right corner to open the editor.
Scroll down to Event Details.
Scroll down to find Event Poster and Event Background. You can click the X at the upper right corner of the image to remove it and then upload a new one.
Underneath that, choosing Use event poster as background image will take your poster image, blur it and scale it to fit the page.
You can instead choose to have a solid color as your background image. Select Set background color.
Click the rectangle on the right side to select a color.
Update your settings and the changes will be reflected on the event page.