Scenario: Customer has a portal integrated with CRM 2015 and wants the look and feel of the web application to be like CRM 2015.
Resolution: First thing to use in CRM 2011 days was that CSS was outlined in a folder in the SDK folder. But ever since CRM 2013 is out, this approach is not possible as it is not exposed in CRM CSS via SDK.
As per the official blog link from Dynamics PFE Team mentioned here:
The approach is to use Developer tools in your browser and take out the CSS as required. So, I used the approach mentioned in the above link and apply it to ASP.NET plain website. Here are the results: