Hi Ayush,
Please follow the below steps:
1. Add the css file as a MIME object.
2. Add the the following in the <head></head> tag:
<link rel="stylesheet" href="`mimeURL(~service="itsmobile", ~theme=99, ~language="", ~name="YourCSSFilename.css")`" type="text/css" />
Note: If you have created folders in the MIME Object folder, give the entire path starting from the foldername immediately below the MIME Object folder, until your CSS file. For example:
Then instead of just writing "YourCSSFilename.css", write "styles/all/abc.css".
Let me know.