In the default theme the following CSS allows you to change colors:
#sidebar ul li {
list-style-type: none;
list-style-image: none;
margin-bottom: 15px;
color: red;
}
That will change the color on unlinked text in the sidebar to red.
#sidebar ul li a{
list-style-type: none;
list-style-image: none;
margin-bottom: 15px;
color: red;
}
#sidebar ul li a:hover{
list-style-type: none;
list-style-image: none;
margin-bottom: 15px;
color: purple;
text-decoration: none;
}
And this (copy and paste) CSS will make all sidebar links red. Also, links will change color to purple on hover.
You can change colors how much you want (using color codes or descriptions, more on that over here -> https://www.computerhope.com/htmcolor.htm ). All themes does not have the same CSS classes, but it should be something similar to this.