I am talking about this:

1. Create a logo image of size 130×60 px
2. Upload the image to web accessible location
3. Copy this style sheet
4. Adjust line 6 to set the header background color
5. Adjust line 14 to point to your logo image
6. Upload the style sheet to a web accesible location
7. Enter Jenkins and install the Simple Theme Plugin.
8. Go to Manage Jenkins > Configure System, look for the Theme section
9. Set the URL of theme CSS to point to you style sheet
10. After saving the changes you should see your new style applied