Use your browser developer tools to inspect the element before and after the click event. It's possible (though I really hope not likely) that the blue styling is being added directly by some client-side script, but it probably ought to be defined in a stylesheet.
Inspecting the element after the click ought to reveal the button's new class name, at the very least you should be able to see its style rules and compare that to the stylesheet rules to determine where to look to change the value.
If it is being explicitly set by the code, then look in that page's code file for a reference to that button's ID value and you should be able to turn it up.
