How I use Css only in master page and the content page must not use it

Create two different CSS files.
One for Master Page & another one for Content page.

Make sure. both the CSS have different CSS classes.

-KR

You need to design the page in that way. Define css classes which are also used in Master Page and not in content pages.

You can use Id instead of class for this.

As Id are supposed to be uniqe in DOM, so any css applied to that control id present in master page, will only be added to that id and not to the content pages.

#myMasterPageControl { color: red; }
.myChildPageControl { color: green; }

Div in master page:

<div id="myMasterPageControl">Some Text</div>

Div in content pages:

<div class="myChildPageControl">Some Text</div>
<div class="myChildPageControl">Some Text</div>
<div class="myChildPageControl">Some Text</div>