Change color of text in Recent Posts and Post Categories Widgets

Hi,
if you want to make li background black you can apply this CSS rule in your theme options:

.sidebar li {
background-color: #000;
padding-left: 5px;
}

or if you need to make the links black:
.sidebar a {
color: #000;
}

You can also apply these rules by creating a child theme or enqueue a custom stylesheet or edit the theme stylesheet directly but you may lost whenever the theme updates in this last one.