update dropdown bg color to differentiate it from border color (#415)

Co-authored-by: Panos Christophides <panos@netdata.cloud>

Author: christophidesp

src/theme/dark/colors.js

@@ -9,7 +9,7 @@ const appColors = {
   tabsBorder: rawColors.neutral.ratsbane,
   disabled: rawColors.neutral.tuna,
   disabledBackground: rawColors.neutral.outerSpace,
-  dropdown: rawColors.neutral.ratsbane,
+  dropdown: rawColors.neutral.arsenic,
   elementBackground: rawColors.neutral.outerSpace,
   elementBackgroundHover: rawColors.neutral.tuna,
   mainBackground: rawColors.neutral.gunmetal,