Overriding the css of a theme from a plugin

I am building a menu plugin that utilizes jquery and css to style and interact with a custom menu. The plugin works as intended in the default theme but in most other themes there are lots of issues with style inheritance (mainly with hovers). I managed to get the plugin css loaded after the theme styles using wp_enqueue_style() but attempting to override the theme css (even a direct copy of selectors) doesnt seem to work. Example and github sources are below:

main github repo

current theme style

Example of issue. Mouseover the links on side to see hover style issues

the exact style being applied during hover is this:

.widget li:hover {
	background-color: #2183c4;
	-moz-box-shadow:1px 1px 5px #000;
	-webkit-box-shadow:1px 1px 5px #000;
	box-shadow:1px 1px 5px #000;
	-moz-border-radius: 8px 8px 8px 8px;
	border-radius: 8px;
	color: #fff;
}

Should be the following for the plugin only:

.widget li:hover {
	background-color: none;
	-moz-box-shadow: none;
	-webkit-box-shadow: none;
	box-shadow: none;
	-moz-border-radius: none;
	border-radius: none;
}