[Web] Fixed form fields bg color in dark mode

Signed-off-by: Kristian Feldsam <feldsam@gmail.com>

data/web/css/themes/mailcow-darkmode.css

@@ -175,6 +175,9 @@ pre {
     background-color: #282828;
     border: 1px solid #555;
 }
+.form-control {
+    background-color: transparent;
+}
 input.form-control, textarea.form-control {
     color: #e2e2e2 !important;
     background-color: #424242 !important;