Hi Neeraja,
An entity form is associated with a form in Dynamics, therefore, you can change layout of the corresponding form in form editor, it will reflect on entity form in portal.
If you would like to only change layout of form in portal, to apply own css to entity form or entity list, we can take following steps.
1. Find the web page record that we added entity form or entity list.
2. Open the associated localized content record.

3. Add custom css in Advanced > Custom CSS

Result:

In addition, it may takes several minutes to sync modification, you can force portal to refresh in https://make.powerapps.com/
Power Apps > Apps > Portal > Edit > Browse website.

Regards,
Clofly