Theme Tweaker grabs all your existing theme colors and allows you to modify them either by typing the color codes or by using color picker. The plugin offers you another feature to change theme colors in bulk, like you can invert all colors, use grey scale, sepia style colors or you can generate random colors in a click etc.
First install and activate Theme Tweaker plugin. Upon activation visit your admin area Dashboard -> Appearance -> Theme Tweaker page, scroll down and you will see a simple table showing all colors active on your current WordPress theme. Now you can use the color picker and invert your own colors.
The plugin offers you a reset option for situations when you mess up with all your theme colors. Not only this much but you can it also shows you the original colors your theme has. After you complete up inverting colors you can preview them and then enable them for all your users and visitors.