how to change "update my current view" icon for custom skin in v8?

We created a custom skin by copying the "serenity" skin files and making minor color changes to the skin.properties file. Note that the "serenity" skin has a white "update my current view" refresh icon in the upper-right corner.

When we deployed our custom skin, this icon was now black. It is difficult to see on top of our color scheme (see attached).

How can we modify this icon so that it is white like in the serenity theme?