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") };