If you've noticed your rule lines are not showing up in your email template, chances are the rule line is just displaying the default color. The default color if not selected is no color.
To select the color and size for a rule line just follow these instructions:
1. Hover over the line when you're building your email template
2. Select Edit Horizontal Rule
3. Click on the color picker icon
4. Deselect the "None" checkbox and select a color