All of the themes I've been looking at use common CSS. I don't see any usage of LESS to generate the CSS. Hardcoding CSS into a theme seems to me to be as primitive as hard-coding a website itself without the use of a CMS or framework. Why don't more theme developers build their CSS with LESS or similar tools?
For example, why hardcode colors and their various hues throughout a stylesheet when a single color reference up-top can be changed to modify the entire site? And in plugins that include CSS (was just looking at Q&A from WPMUDEV) why hardcode the width of text fields in-line in CSS when a high level variable setting would be more efficient for later changes?
This is all subject to personal preferences about whether to use tools and which to use, and there are many levels of development expertise. I'm just surprised that this concept hasn't become a defacto part of the WP culture, like the template hierarchy, hooks, etc.
My personal frustration with this is that I might like a theme but not the chaos in the .css files. I can LESSify the CSS into a child theme, but when the parent theme is updated, reconciling the author's new CSS with the generated CSS takes a LOT of time - in essence, this operation creates a permanent fork of the theme and precludes updates.
Of course, some plugin/theme developers could be using LESS or SASS or something else to produce their CSS, and just not including those source files in their packages. But my guess is that this is not the case.