Custum css webpart for sharepoint online 2013

Hi,

I would like to do some styling using css at a document library. 

I am using sharepoint online 2013, office 365. 

Is this possible, and how should i implement the css file into sharepoint, I have tried using the webpart content editor as reference to the css file, and storing the css file in site assets.  

My css:

. ms-WPHeader td {
     background-color: Blue;
}


.ms-standardheader ms-WPTitle a {
     color: White;
}

thx.