Some custom user styles I created to clean up the latest Gmail theme from Google. Removes some annoying whitespace, adds hover effect on the inbox, removes ads.

gistfile1.css

// List
tr.zA > td{
   padding-top: 5px !important;
   padding-bottom: 5px !important;
}

// Mails
tr.zA:hover{
   background-color: #FFFBC4 !important;
}

// Ad
.mq{
   display: none !important;
}

// Navigation
.TN{
   padding: 0 !important;
   margin-bottom: 5px !important;
}

// Buttons
.J-Zh-I{
   padding: 4px 9px !important;
}

// Buttons
.J-Zh-I-Js-Zq.J-Zh-I-JW, .J-Zh-I-Js-Zq.J-Zh-I-JO, .J-Zh-I-Js-Zq.J-Zh-I-Je{
   padding-left: 9px !important;
   padding-right: 9px !important;
}

// Ads
.oM, .OH0apf{
   display: none !important;
}

// Mail tab
.gE.hI:hover{
   background-color: #FFFBC4 !important;
}