Changing Style Sheet javascript

One guess would be the missing .css property, another would be the fact that onclick is not a function, but a result from invoking one:

Make all of .css a string and assign functions to onclick:

style1.onclick = function () { swapStyleSheet("default.css") };
style2.onclick = function () { swapStyleSheet("dark.css"); };

Transform "default".css into "default.css". Do the same for dark.css.

Then onclick takes a function as a value.

style1.onclick = function () { swapStyleSheet("default.css") };
style2.onclick = function () { swapStyleSheet("dark.css") };