First I would recommend that you use a custom button or link to inject jquery. Have a look on this how you can load custom JS:
Changing the color of a custom button
The button/link will have no function. It is just use as a workaround to hook custom scripts in. Later, I probably would make it invisible with jquery.
If you have jquery up and running, use it together with Firebug to figure out what is needed.
You need to learn and understand at least basic concepts of HTML, CSS, jquery and firebug to get it done. But then it's very easy. Also be aware that that this workaround is not officially supported an SFDC may change things unannounced and you will have to fix your code. So you need to know what you are doing. You can't just paste a snippet in which you don't understand. In the last years we have seen only very rare changes to CSS/HTML and such workaround where very stable. But Saleforce plans to overhaul the UI. They keyword here is Aura. I would expect some adaptions in the next 1 or 2 years to become necessary.