There are couple of a Chrome Extensions, Stylish and StyleBot which allow for designing of themes and style individual elements.
While Stylish relies on userstyles for custom styles, Stylebot has a an inbuilt sharing tool for share and fetch custom style sheets for most sites.
For this specific Stack Exchange, there's a userscript which allows you to swap stylesheets for any Stack Exchange site from any Stack Exchange site.

Note that the stylesheets are swapped after the page has loaded, so there will be a momentary flash where the original stylesheet is shown and then the new one is swapped. If you have a fast enough connection, this will probably be not noted by you.
Finally, recent Chrome update allows you to install userscripts & extensions via 1 click only from the Chrome WebStore.
To install the above linked userscript, you'll have to download/save the .user.js file, open chrome://chrome/extensions/ (or Click on Chrome Menu → Tools → Extensions ) & drag the *.user.js file to the Extensions tab. After that, you'll get the prompt to install the extension.
1StackExchange Theme Switcher – Sathyajith Bhat – 2012-07-31T05:15:28.267
Although it is not the way I asked to change the theme, but it actually a perfect answer so please post it as answer to vote up and accept it :) – Wazery – 2012-07-31T05:24:47.653
1done, added as an answer @wazery – Sathyajith Bhat – 2012-07-31T05:41:42.850