Hi Jalligama
There is no way to change a controls background colour using rules or conditional styles as SmartForms do not have a Focus event for controls.
I suggest you look at modifying the Theme (CSS) you are using to modify the focus style for controls. If you are not familiar with K2 Themes then have a look at the K2 SmartForms Developer Guide
As an example, I changed the following entry for the SharePoint 2013 theme (CSS file):
.theme-entry .SFC.SourceCode-Forms-Controls-Web-TextBox.focus,
and set the background colour to green (background: #40FF00;), which resulted in the textbox having a green background when I click into it (reverts back to normal when I click out)
Alternatively you could insert some javascript into your form to try and do the same thing (see
http://www.aspsnippets.com/Articles/Change-TextBox-Background-color-on-Focus-using-JavaScript-jQuery-CSS.aspx)