Match the combobox search font to the items and space it evenly

The search field now uses the font size of the items under it, and the
space around it comes from two variables: combobox-search-gap for the top
and bottom, and combobox-search-gap-x for the sides.

core/scss/_variables.scss

@@ -1406,6 +1406,8 @@ $dropdown-header-padding-y: $dropdown-padding-y !default;
 $dropdown-max-width: 25rem !default;
 $dropdown-scrollable-height: 13rem !default;
 $combobox-menu-max-height: 300px !default;
+$combobox-search-gap: 0.375rem !default;
+$combobox-search-gap-x: 0.5rem !default;
 $dropdown-item-gap: 0.5rem !default;
 $dropdown-header-padding-bottom: 0.25rem !default;
 $dropdown-menu-columns-gap: 0.25rem !default;

core/scss/ui/forms/_combobox.scss

@@ -119,11 +119,13 @@
 }
 
 .combobox-search {
-  padding: 0 calc(0.75 * #{$dropdown-padding-y}) calc(0.75 * #{$dropdown-padding-y});
+  padding: 0 $combobox-search-gap-x $combobox-search-gap;
+  margin-top: calc(#{$combobox-search-gap} - #{$dropdown-padding-y});
 }
 
 .combobox-search-input {
   width: 100%;
+  font-size: inherit;
 }
 
 .dropdown-item-label {