Line number color fixes (#169)

Author: luah5

Sources/CodeEditTextView/Controller/STTextViewController.swift

@@ -127,7 +127,7 @@ public class STTextViewController: NSViewController, STTextViewDelegate, ThemeAt
 
  rulerView = STLineNumberRulerView(textView: textView, scrollView: scrollView)
  rulerView.backgroundColor = useThemeBackground ? theme.background : .clear
- rulerView.textColor = .systemGray
+ rulerView.textColor = .secondaryLabelColor
  rulerView.drawSeparator = false
  rulerView.baselineOffset = baselineOffset
  rulerView.font = NSFont.monospacedDigitSystemFont(ofSize: 9.5, weight: .regular)