Add CSS class into HTML

Is it possible to insert CSS file into HTML separately? like create a new CSS file and then insert the whole file into my current HTML.

In my case I used "a" tag in the nav bar and I want to make a linkable image on the website, so I need to use "a href" to link pages, however the "a" tag were linked, but in the same form as the bar, which is a box (I used box shaped for the nav, instead of just hyperlink).

like this:
a:link, a:visited {
display: block;
width: 230px;
font-weight: bold;
font-size:16px;
color: #FFFFFF;
background-color: #60C;
text-align: center;
padding: 4px;
text-decoration: none;
text-transform: capitalize;
}

a:hover, a:active {
background-color: #FF6;
color:#000
}