Improve consistency of dropdowns with USWDS, other fields (#263)

**Why**: Backports styling from brochure site, and makes it so that form fields share common background and border styling, and inherits more from USWDS (specifically the arrow graphic).

CHANGELOG.md

@@ -5,7 +5,8 @@
 - Add Login.gov-specific component configuration. ([#258])(https://github.com/18F/identity-style-guide/pull/258)
   - [Identifier component](https://designsystem.digital.gov/components/identifier/)
   - Link colors on dark backgrounds
-- Overlay: Updated visual appearance. ([#260](https://github.com/18F/identity-style-guide/pull/260))
+- Overlay is now shown with a lighter backdrop color. ([#260](https://github.com/18F/identity-style-guide/pull/260))
+- Form Dropdown is now more visually consistent with other form fields. ([#263](https://github.com/18F/identity-style-guide/pull/260))
 - Form hint text is now shown with an italicized style and increased vertical margins. ([#262](https://github.com/18F/identity-style-guide/pull/262))
 
 ## 6.2.0

src/scss/components/_inputs.scss

@@ -14,7 +14,8 @@ $checkbox-radio-size: .875rem;
 $input-select-margin-right: 1;
 
 .usa-input,
-.usa-textarea {
+.usa-textarea,
+.usa-select {
   @include u-border-color('primary-light');
   font-weight: font-weight('bold');
   background-color: color('primary-lightest');
@@ -39,24 +40,6 @@ $input-select-margin-right: 1;
   border-top-right-radius: 0;
 }
 
-.usa-select {
-  @include u-border-color('primary-light');
-  @include add-background-svg('arrow-down-filled');
-  font-weight: font-weight('bold');
-  background-position: right .75rem center;
-  background-size: units(1);
-  border-radius: radius('md');
-
-  &:focus {
-    @include disable-default-focus-styles;
-    box-shadow: roundable-focus-outline-box-shadows();
-  }
-
-  &.usa-input--error {
-    @include u-border-color('error');
-  }
-}
-
 .usa-label {
   font-weight: font-weight('bold');
 }