Improved legibility of REST Mock interface and various fixes (#266)

Reviewed-on: #266
Reviewed-by: Mikolaj Widla <widlam@noreply.example.com>
Co-authored-by: Adam Bem <adam.bem@zoho.eu>
Co-committed-by: Adam Bem <adam.bem@zoho.eu>

Frontend/src/style.css

@@ -26,4 +26,8 @@
 
 .file-selector {
     @apply block file:border-none file:font-sans file:text-base file:hover:brightness-110 file:py-2 file:px-4 file:h-full file:w-fit file:rounded-full file:bg-gradient-to-r file:from-blue-400 file:to-sky-300  file:dark:text-white file:dark:from-sky-600 file:dark:to-sky-800 file:hover:bg-blue-400 w-fit rounded-full text-sm text-gray-900 border border-gray-300 cursor-pointer bg-gray-50 dark:text-gray-400 focus:outline-none dark:bg-gray-700 dark:border-gray-600 dark:placeholder-gray-400
+}
+
+.text-field-error {
+    @apply shadow-[0px_0px_10px_0px_rgba(255,0,0,1)];
 }