I've built some pretty complicated styles and always used the same approach. Nice clean syntax and good commenting in the css file(s). Like this:
/***** /Products/Edit ******/
#productsContainer { background-color: #99bbff; }
#productsContainer fieldset.edit { margin: 1em 0 .2em 0; background-color: #dadada; }
#productsContainer fieldset.edit label { display: block; }
/***************************/
If you stay organized, control the efficiency of your styling, and optionally use something like CleverCSS: https://github.com/dziegler/clevercss, you can manage your styles without complicated overhead and tools that can break. CleverCSS example below:
/***** /Products/Edit ******/
#productsContainer
background-color: #99bbff
fieldset.edit:
margin: 1em 0 .2em 0
background-color: #dadada
label:
display: block;
/***************************/
But yeah, optionally you can use something like WebStorm (http://www.jetbrains.com/webstorm/) to edit your javascript, html, css, etc. Or wait for Expression Web to be ready (I'd guess less than a year until it becomes compatible with MVC).