DEV: Refactor dark-light-diff usage in stylesheets

- Adds new `$danger-low-mid` SCSS variable
- Removes `dark-light-diff` outside variables and color transformations

app/assets/stylesheets/common/foundation/color_transformations.scss

@@ -64,6 +64,12 @@ $highlight-high: dark-light-diff($highlight, $secondary, -50%, -10%) !default;
 
 //danger
 $danger-low: dark-light-diff($danger, $secondary, 85%, -64%) !default;
+$danger-low-mid: dark-light-diff(
+  rgba($danger, 0.7),
+  $secondary,
+  50%,
+  -60%
+) !default;
 $danger-medium: dark-light-diff($danger, $secondary, 30%, -35%) !default;
 
 //success