Hello Hideki. I added some more specific selectors and additional rules to target the drop down and the textarea as well. This is what I applied to your form:
body div#gform_wrapper_1 div.gform_body input[type="text"],
body div#gform_wrapper_1 select,
body div#gform_wrapper_1 textarea {
background-color: rgb(114, 132, 156);
color: white;
}
When applied to your form, it looks like this:
