Converted primary color use to css variable

- Removed all existing SCSS usage of primary color.
- Cut down custom styles injection to just be css vars.
- Reduced button styles so default button is primary.
- Updated button styles to lighten/brighten on hover & active states even
when a custom color is set.
- Removed unused scss color vars.
- Updated default BookStack blue to achieve better accessibility.

resources/views/search/all.blade.php

@@ -182,7 +182,7 @@
                         </table>
 
 
-                        <button type="submit" class="button primary">{{ trans('entities.search_update') }}</button>
+                        <button type="submit" class="button">{{ trans('entities.search_update') }}</button>
                     </form>
 
                 </div>