Change color of Previous Post and Next Post

Add these rules:

#paging a {
    background-color: blue;
    color: white;
}
#paging:hover a {
    background-color: red;
    color: yellow;
}

The first rule is for the plain, non-hovered, state; the second rule is for the hovered state. The color property is the text color. Change the values as you like. Instead of the reserved words for the colors, you can also use the hex values instead. There’s a color picker here if you need to look up the hex code.