You can change background color for your fields from the Advanced field styling options in the Theme Customization page.
In Theme Customization settings. Change to the theme you want to customize from the select box at the top and click Show Advanced Options button to enable Advanced customization features.
Using transparency in Field Background
The color picker allows you to use transparency in your colors. To use it, simply use the slider at the bottom of the color picker. Left to right, sets the transparency from (completely transparent) 0 to 100% (completely opaque).