How can I change the background color of the fields in contact form manager plugin?

You can place a css class in the form content at the beginning of the form (in the HTML section) like given below.
<style type="text/css">
.xyz_cfm_style
{
background-color : white;
}
</style>

(Note: If you are copy-paste the above code, please change the quotes “” in style type=”text/css”. The quotes in the content is different from original quotes.)

Here xyz_cfm_style is the class name.

Now please go to the edit element section and enter the classname to the fields.

In some cases, your default style may override the custom style. In such a condition, please use the code as

<style type="text/css">
.xyz_cfm_style
{
background-color : white  !important;
}
</style>

In some server, the style tag entered in the form section will disappear when saving it.

In such a situation, you can add the style in the style.css (default style) file of wordpress installation.