You can add additional style definitions to an HTML caption in your donation form. If you'd like everything to be the same color (form background, buttons, text input fields), then insert this code into the plain text editor of your HTML caption:
<style>
#processForm td, #processForm input, #processForm select {background:#F6D5CA}
</style>

 
Alternatively, if you'd like to retain the background color of the section headers and buttons as defined in your custom style sheet, use this code instead:
<style>
#processForm td.FormInput, #processForm td.FormLabel, #processForm td.requiredIndicator, #processForm td.holdsErrorMsgTable{background-color:#F6D5CA}
</style>
 
If you'd like to add a border to your donation form, add this code to your style definition:
#processForm {border-style: solid; border-width: 1px; border-color: #868686}
 
For example, if you want everything to be the have the same background color and a border, your code would look like this:
<style>
#processForm td, #processForm input, #processForm select {background:#F6D5CA}
#processForm {border-style: solid; border-width: 1px; border-color: #868686}
</style>