I might offer a high-contrast stylesheet as an option (if the contrast of the default colour scheme is potentially a little low for some), but other than that, it's up to them.
Fwiw™, I suspect the number of impaired users using their own customised stylesheet is fewer than you might think.
Assistive Technology (AT) typically provides those who use it with easy access to stylesheet overrides and high-contrast settings.
Opera does a good job of providing some worthwhile accessibility features directly in the browser itself. However, for the sake of design control, I might still be tempted to offer a styleswitcher with an alternative, high(er)-contrast (but, still sympathetically designed) stylesheet.
I tend not to provide type size widgets on-site. Imo, it's optimal that users are ushered into learning and using those facilities built into the browser.