Form elements don’t have their background color directly configurable, as they are designed to use the same background as the content, with the last 3 accent colors used for the border colors (to create the engraved effect).
If desired, this can be overridden with custom CSS:
input[type="text"], input[type="password"],
input[type="email"], input[type="color"],
input[type="date"], input[type="datetime"],
input[type="datetime-local"], input[type="month"],
input[type="number"], input[type="range"],
input[type="search"], input[type="tel"],
input[type="time"], input[type="url"],
input[type="week"], textarea, select {
background-color: ...;
}