If you hover with your mouse over a column which is sortable, I'd like to show a light green color as indicator that the column is actually sortable.

The arrow should point downward (the default sort direction for a column) and feature color #11 from the style guide: http://wiki.magnolia-cms.com/display/UX/Style+guides+for+the+Magnolia+shell#StyleguidesfortheMagnoliashell-Messageslistandgenerallistofitems