Add dropdown caret to time owner dropdown on Date and Time Settings

- Use background-image strategy to add caret to dropdowns
- Standardize implementation by including mixin to dropdown__button
  class and removing styled pseudo element

Resolves ibm-openbmc/dev#478

Tested: Running locally on Chrome only. Ran into network request
        errors for newly added svg assets when mounting overlay

Signed-off-by: Yoshie Muranaka <yoshiemuranaka@gmail.com>
Change-Id: I714c2515282676f368eafb2560177d2935680e23
diff --git a/app/common/styles/base/forms.scss b/app/common/styles/base/forms.scss
index 73c25c1..eb633d4 100644
--- a/app/common/styles/base/forms.scss
+++ b/app/common/styles/base/forms.scss
@@ -92,12 +92,15 @@
 //Custom select
 select{
   border-radius: 0px;
-  height:auto;
+  height: auto;
+  padding-right: 0.5rem; //override inherited Foundation style
   @include fastTransition-all;
   @include bgImage__arrowDown-primary;
   &:focus {
     border-color: $lightbg__accent;
-    @include bgImage__arrowDown-accent;
     box-shadow: 0 0 0;
   }
+  &[disabled] {
+    @include bgImage__arrowDown-disabled;
+  }
 }