Fix Content Migration's Select Content button disabled bg

Closes ADMIN-1347

Test Plan:
* As an admin
* In a course -> settings -> Import Course Content
* Import a common cartridge file
* Select "Select Content" instead of "All Content"
* Import
* After the upload / processing is done, click the
  "Select Content" button on the page
* The "Select Content" button on the modal that appears
  should have a blue background that appears disabled
  until you select an item from the tree to import

Change-Id: Idd139711aae36f4a5edf7b8953716fd722f9754f
Reviewed-on: https://gerrit.instructure.com/163774
Tested-by: Jenkins
Reviewed-by: Mysti Sadler <mysti@instructure.com>
Reviewed-by: Ed Schiebel <eschiebel@instructure.com>
QA-Review: Ed Schiebel <eschiebel@instructure.com>
Product-Review: Dan Sasaki <dsasaki@instructure.com>

app/stylesheets/bundles/content_migrations.scss

@@ -200,3 +200,7 @@ i.course_select_warning {
   padding-#{direction(left)}: $ic-sp;
 }
 
+#selectContentBtn.btn-primary {
+  background: var(--ic-brand-button--primary-bgd);
+  color: var(--ic-brand-button--primary-text);
+}