how to change focus color in css code example

Example 1: css underline color

u
{
	text-decoration: underline;
  	text-decoration-color: red;
}
example of use: (in html)

<p>The word <u>CAT</u>, is underlined </p>

Example 2: css focus change color

<style>
.dabutton:focus {
    background-color:yellow;
}
</style>
<button class="dabutton"></button> // <-- usage