SharePoint Design Menu Options

Hi, I am very new to SharePoint 2013 administrator. I do not have any web development background. 

A user has requested they want to adjust the css styles to be a number of different color combos for their tables on their subsite.

How they got there:

• They type in their site url.
• Click the Edit (in the upper right)
• Click a title in their table
• Select Design from the menu tab
• Select Style choices

They want to customize the styles pulldown list for their site.

Can anyone point me to a solution?