how to set multiple css for 2 different pages /controls in one website

First question is why different stylesheets for different pages? Should be no reason you can't have that all in one stylesheet and simply reference the class you want in your control.

If your class is "btn1" and the class assigned to the control (that you may or may not have access to) is "filterButton btn btn-default", you'd have to modify your stylesheet to be something like this:

.btn1, .filterButton .btn .btn-default { background-color:red; color: white; }

Hi Brendan. Thanks for your help. just i am confused here.
example: website/page1.aspx if i want to change the button color of page1.aspx to blue (as it is ) code is :

.sendToFriendPanel input[type=submit], .LogonDialog input[type=submit], .LoginPanelPasswordRetrieval input[type=submit], .LongButton, .EventManagerRegistration .EventManagerRegButton, .filterButton, .WishlistTable .btnContinue input, .CartTable input[type=submit], .CntRatingSubmitBtn, .commentView .btn-primary, .SubscriptionButton, .ProductFilter .btn, .FormButton, .examplesToLeft input[type=submit], .CMSSearchDialogSearchButton, .ChangePasswordTable input[type=submit], .RegisterButton, .my-profile-panel .btn, .Account .btn, .myAccount .btn, .datetime-ui-datepicker .btn, .ContentButton, .CartStepBody .btn, .CartStepFooter input, .FilterSearch .btn, .DonateWebPart .btn, .ExamplesContent .form-minilogon .btn, .MyMessages .btn, .ContributionsEdit .btn, .registration-form .btn, .SearchDialog .btn, .abuse-report .btn, .PollVoteButton, .membership-req-data .btn, .CntRatingContent .btn, .unsubcription-request .btn, .Search .btn, .new-blog .btn, .ForumExtendedSearch .btn, .message-edit .btn, .new-subscription-form .btn, .send-message .btn, .forum-search .btn, .search-dialog .btn, .change-password .btn, .WizardButton, .PageContent .btn-actions .btn, .header-panel .btn, .MassAction .btn, .Forum .btn, .UniSelector .btn, .EventManagerRegistration .btn, .WizardButtons .btn, .NotificationSubscriptionButton .btn, .message-view .btn, .ExamplesContent .form-filter .btn {background-color: blue;}

website/page2

i want to change the button color to green :

.sendToFriendPanel input[type=submit], .LogonDialog input[type=submit], .LoginPanelPasswordRetrieval input[type=submit], .LongButton, .EventManagerRegistration .EventManagerRegButton, .filterButton, .WishlistTable .btnContinue input, .CartTable input[type=submit], .CntRatingSubmitBtn, .commentView .btn-primary, .SubscriptionButton, .ProductFilter .btn, .FormButton, .examplesToLeft input[type=submit], .CMSSearchDialogSearchButton, .ChangePasswordTable input[type=submit], .RegisterButton, .my-profile-panel .btn, .Account .btn, .myAccount .btn, .datetime-ui-datepicker .btn, .ContentButton, .CartStepBody .btn, .CartStepFooter input, .FilterSearch .btn, .DonateWebPart .btn, .ExamplesContent .form-minilogon .btn, .MyMessages .btn, .ContributionsEdit .btn, .registration-form .btn, .SearchDialog .btn, .abuse-report .btn, .PollVoteButton, .membership-req-data .btn, .CntRatingContent .btn, .unsubcription-request .btn, .Search .btn, .new-blog .btn, .ForumExtendedSearch .btn, .message-edit .btn, .new-subscription-form .btn, .send-message .btn, .forum-search .btn, .search-dialog .btn, .change-password .btn, .WizardButton, .PageContent .btn-actions .btn, .header-panel .btn, .MassAction .btn, .Forum .btn, .UniSelector .btn, .EventManagerRegistration .btn, .WizardButtons .btn, .NotificationSubscriptionButton .btn, .message-view .btn, .ExamplesContent .form-filter .btn { background-color: green }

how to accomplish this with 1 CSS file ?

classname for both button uses: class=" FormButton btn btn-primary "

You need to distinguish these items by creating a css class for blue and one for green. Then go to your elements and add the class directly to the items. Set the CssClass property to "mypropGreen" or "mypropBlue". This will allow for the same elements to carry different styles, otherwise, if you are using the classes above, that are the same, you won't be able to control both, one will just override the other. You can keep them all in one file if you create new css classes and then attach them.