Set scrollView background color (CodeEditApp#113)

# Description 

Updates the scroll view's background color to the theme's background
color to avoid ugly edges on scroll.

**Before**


https://user-images.githubusercontent.com/35942988/211999177-5a11f56b-3cfd-44d2-95f5-b866ae276e98.mov

**After**


https://user-images.githubusercontent.com/35942988/211999157-b3926ec5-0115-4545-9b4a-728f7f79771f.mov

# Related Issues

N/A

Sources/CodeEditTextView/STTextViewController.swift

@@ -211,6 +211,7 @@ public class STTextViewController: NSViewController, STTextViewDelegate, ThemeAt
         rulerView?.separatorColor = theme.invisibles
         rulerView?.baselineOffset = baselineOffset
 
+        (view as? NSScrollView)?.backgroundColor = theme.background
         (view as? NSScrollView)?.contentView.contentInsets.bottom = bottomContentInsets
 
         setStandardAttributes()