There's a great tool you can use for generating an ActionBarSherlock compatible Theme. Go to the following link and initialize it with the following settings:
- Style compatibility: Sherlock
- Base theme: Dark
- ActionBar style: Solid
This will remove the line below the ActionBar and allow you to choose a solid color for the ActionBar itself. If you simply wanted to change the color of that line, you can set the ActionBar style to Transparent and change the Accent color (this is the color chosen for that line)