HTML/JS/CSS styled buttons, help please

I am adding password protection to a Muse page using the following HTML code.

<SCRIPT LANGUAGE="javascript">

                <!--- Hide from tired old browsers

                var nifty_little_window = null;

                function gateKeeper() {

                nifty_little_window = window.open('gatekeep.html', 'theKeeper',

                'width=350,height=200,resizable=1');

                }

                // End hiding --->

                </SCRIPT>

<form>

<input type="button" value="Member Log In" onClick="gateKeeper()"

</form>

The full details of this rather useful JS is here Cut and Paste JavaScript- Gate Keeper Password Protect

This opens another page asking for a password, all works well except the buttons are generic, grey rectangles —ugly! I found all sorts of resources for generating CSS to make pretty buttons but I don't know what to do with the code (which is why I am a Muse user not a DW user!). I have been able to find out that CSS can be implemented in three ways, external, internal and in-line. As this is one button on a master page the in-line option looks like the way to go. I have pasted the CSS code into the HTML editor but to no avail, it ether does nothing or just appears as text. Really simple instructions for a HTML/CSS idiot would be much appreciated.