Question: Is it possible to change the background color of a team site, and a web part as well? are we able to design a web part with rounded corner fx?

Answer: 
yes we need to override the css having the background color and u will be able to change the color.
The background color of the sites can be changed via CSS. In case you don't want to write your custom CSS, you can also apply any of the in-built themes to your SharePoint sites. Theme can be changed using following steps:

* Go To Site Settings -> Change the Look (Under Look and Feel Section)
* Select any of the available theme on the page, change the colors, images etc. based on your requirement and click on Try it Out to see a preview.
	+ If you like the preview, then click on Yes, Keep it and your site would acquire the new theme.

The corners of a web part can be made rounded using a CSS property called:

```
border-radius:<<Pixel Units here>>

```