How do i change theme color to black

I have tried adding black.css to color, even tried changing existing css but it does not change on my site??

 

There is no need to add additional black css. Themes default color is black.
Go to
Appearance > Theme options > styling options > Theme stylesheet
And select default color from there.

If you want to edit it, Then you can edit it through style.css.

Note: If your changes are not showing at the theme then deactivate cache plugin (if installed).