Hi Abdelfattah,

You can have a look at the following K2 documentation: https://help.k2.com/onlinehelp/K2smartforms/DevRef/4.7/default.htm#Creating_a_Custom_Theme.html%3FTocPath%3DCustom%2520Themes%2520%7CGetting%2520started%2520with%2520Custom%2520Themes%7C_____1
Click to expand “Responsive Custom Theme” section, and there you will see the explanation about a bunch of css files, which are part of the theme. And in one of the files, i.e. Theme.css, you can see a lot of regions, which clearly separate K2 SmartForms blocks, to which the styles are applied. This should kind of give you an idea of which class needs to be used for a certain control.

HTH.