The default setting is that there is no background color for text boxes. This allows the text to be freely placed in any location.
To set a background color, you have to set a background color in the right column in the "Text field" area. Additionally, you can choose an opacity value of between 1% and 100%.
This will cause the text field to be filled in with the selected color.
Tip: If you set the width of the text box to "0", the width will automatically adjust to the text length and no line break will be forced.