Enable input focus state customization

- Add new mixin to generate and customize focus state as needed
- Adds variable to set default color
- Include clear disclaimer about customizing this—it's about users', so
don't go making everything bright red and expect them not to be
confused or alarmed.

Relevant issues: #2742, #4185, #7942, #8171, #8610, #9044

less/mixins.less

@@ -544,3 +544,25 @@
     background-color: @background-color;
   }
 }
+
+// Form control focus state
+//
+// Generate a customized focus state and for any input with the specified color,
+// which defaults to the `@input-focus-border` variable.
+//
+// We highly encourage you to not customize the default value, but instead use
+// this to tweak colors on an as-needed basis. This aesthetic change is based on
+// WebKit's default styles, but applicable to a wider range of browsers. Its
+// usability and accessibility should be taken into account with any change.
+//
+// Example usage: change the default blue border and shadow to white for better
+// contrast against a dark gray background.
+
+.form-control-focus(@color: @input-border-focus) {
+  @color-rgba: rgba(red(@color), green(@color), blue(@color), .6);
+  &:focus {
+    border-color: @color;
+    outline: 0;
+    .box-shadow(~"inset 0 1px 1px rgba(0,0,0,.075), 0 0 8px @{color-rgba}");
+  }
+}