have you tried putting it anywhere?
and what was the result?
just be more adventurous, and try things.
if the results don’t show, try to reload the site in the browser, or refresh the browser cache (ctrl f5).
if you make a backup copy of the working theme files before you start editing, there is not much you can break; especially not with changing styles.
(personally, i would put new styles closely after the related styles in the style.css; there are already some #branding styles.)
also, if the position of the new style in the style sheet would be important, most helpers here would clearly state where to add the styles.
important:
with style.css of TwentyTen, just be aware that the last about 100 lines are for the @media print
; any style for the screen needs to be added before that.