You can add CSS styles to customize the look, as with anything else on your WordPress website. How you do that depends upon your theme and WordPress setup. Some themes have a CSS editor in options. Or, developers use a child theme and edit the style.css file. Or, the JetPack plugin provides a nice CSS Stylesheet Editor.
You need to add CSS style rules, and examine the HTML classes to alter using a tool like the Google Chrome DevTools.
For example, here are style rules that will change the color of the toggle bar to yellow, red for a mouse hover, and keeping white for the toggle item background:
.omsc-toggle {background: #ffff00;}
.omsc-toggle:hover {background: #ff0000;}
.omsc-toggle-inner {background: #ffffff;}
