Font changing and page color background changing


I have 2 question

1.I was following your vidow for changing the fonts, i did step by step(like 30 times) and its not changing the font, please let me know why its not changing.
2.I need to change the white background in the pages to black, in the color part that i change colors to the site i cant find the one that change the color for the pages.

Please let me know