Go to the template edition to set your background in the form. In the Appearance tab, go to Design. Here you can upload a background. The background-size is not specified because you can open the form on devices with different display sizes. If you upload a background with a large header, the background elements on the phone may be directly under the text, making it difficult to read.
The background should be pretty neutral; you can additionally set its brightness and blur with sliders. The proportions should correspond to the dimensions 1680 × 1050 px.
