Hi Jaspal, is there any custom css handler in your application such as less/sass compiler? They might be causing your css to disappear.
One way to add any css to the page is by using Head Html code webpart within this page's template, then reference your style-sheet via the following in the html code section.
<link href="~/CMSPages/GetResource.ashx?stylesheetname=Main" type="text/css" rel="stylesheet" />
<link href="~/CMSPages/GetResource.ashx?stylesheetfile=~/..../file.css" type="text/css" rel="stylesheet" />
I hope this helps.