a { 
    border-bottom: 2px solid transparent; /* <- here */
    transition: border-bottom 1s;
    text-decoration: none; /* I added this for clarity of effect */
}
a:hover {
    border-color: green; /* <-- just change the color instead */
}
body {
	background-color:gray;
	background-image:url("https://evilguild.neocities.org/misc/flames.jpg");
font-size: 25px;
color:#00FF00;
text-shadow: #00FF00 0 0 10px, #000000 5px 5px 5px;

}
img {
    transition:transform 0.25s ease;
}

img:hover {
    -webkit-transform:scale(1.5); /* or some other value */
    transform:scale(1.5);
}
a {
	color: #00FF00;
	text-shadow: #000000 0 0 10px, #00FF00 5px 5px 5px;
}