frontend: fix background color of the dropdown button on mobile

In darkmode the dropdown button was shown in white, should have
same color as the input field which is dark in darkmode.

Author: thisconnect

frontends/web/src/components/forms/input-with-dropdown.module.css

@@ -113,7 +113,7 @@ input.inputField::placeholder {
 
 .dropdownTrigger {
     align-items: center;
-    background-color: #fff;
+    background-color: var(--background-secondary);
     border: 1px solid var(--background-quaternary);
     border-left: hidden;
     display: flex;