Added guidance about the syntax highlighting theme

We don't have any guidance, and we want to keep our images consistent
One way to do this is to ensure we all capture screenshots with the same
syntax highlighting theme

doc/development/documentation/styleguide/index.md

@@ -1151,7 +1151,8 @@ When you take screenshots:
 - **Review how the image renders on the page.** Preview the image locally or use the
 review app in the merge request. Make sure the image isn't blurry or overwhelming.
 - **Be consistent.** Coordinate screenshots with the other screenshots already on
-  a documentation page for a consistent reading experience.
+  a documentation page for a consistent reading experience. Ensure your navigation theme
+  is **Indigo** and the syntax highlighting theme is **Light**. These are the default preferences.
 
 ### Add callouts