Most websites do not restyle every element. For example, if you go into your browser and set the default color to be bright pink, you will find 1-in-10 websites won't set their own foreground color (because they know the default is black and forget to set it) and so the text will be pink. (I even started collecting some! https://gist.github.com/mrspeaker/1846b6b0e12c76f7eeb2)
If you changed the default styles then millions of websites would look really weird.
You can set your own defaults though and test it out!