You can easily post things in your sidebar. If you don't like the prefab widgets, just put it in a text widget:
As for changing the look, you can't, other than selecting among the prefab themes we have here OR learning CSS thoroughly, purchasing the CSS upgrade, and writing the changes you want. You cannot paste in code or themes you got from outside.
Do NOT buy the upgrade if you don't know what you're doing. There's no support and you are completely on your own.