I just created a tutorial on how to How to Center your Blog's Header. Now, I'll show you how to center your Adsense leaderboard by editing your blog's HTML code.
First, you have to know your Adsense leaderboard's widgetID.
You can find your Adsense leaderboard's widgetID by pointing your cursor to the widget. See below photo. Mine is Adsense2.
Go to your Template Designer > Edit HTML > Expand Widget templates then click Ctrl+F "your widgetID" (for example Adsense2) then find < div class='widget-content' > and then replace it with this code < div class='widget-content' style='text-align: center;' >. (Make sure to delete the space after < and before >)