Your UI really shouldn't be so unresponsive that the width of an element's scroll bar throws off your design. As for the aesthetics of the scroll bar: it might look ugly to you, but that's what the user expects their scroll bar to look like!

Designing well for web requires a bit of a paradigm shift compared to designing a print document or even a native app. You have to accept that you're going to be a guest in the user's browser, and that means you don't have control of everything. Failing to accept that just leads to frustration, but if you can embrace it and strive to be a good citizen of the user's browser, the resulting symbiosis can be beautiful in its own right!