People want to style things to match their page, exactly because consistency is part of usability. Especially given the limitations of browser form validations, you will absolutely need your own validations in addition to any browser validations you use. But your own validations will look different from the browser-provided ones, at least on some browsers. Which will confuse users, hence decreasing usability.
And this also assumes that the browser validations are in any way usable to begin with. I would argue that they are not, and would require some sane styling to become usable in the first place.
