According to The Collins English Dictionary a theorist is someone who develops an idea or a set of ideas in order to explain something.
Lea Verou, who is described by Smashing Magazine  as loving code and design equally is a Web designer & developer, Author, Speaker and a Research Assistant. Her CSS book for O’Reilly explains solving problems with code as opposed to design .
Currently she is pursuing a PhD at MIT, doing research in Human-Computer Interaction, specifically end-user programming in the context of data-driven web applications.
She is a Research Assistant at Massachusetts Institute of Technology (MIT) and has worked as a Developer Advocate at W3C.
A strong advocate of Web standards and a big supporter of progressive enhancement and graceful degradation. Websites should be usable, if possible, in older browsers she told Scott Gilbertson in a 2013 interview for wired magazine. 
She has given over 60 talks at web development conferences
She is a Web designer & developer founder at Fresset Ltd (December 2008 – December 2011) Almost everything code or design related in the company.
Her website is lea.verou.me
You can follow Lea on Twitter https://twitter.com/leaverou
A selection of her presentation videos can be found at http://lanyrd.com/profile/leaverou/video/
Verou, Lea (June 2015). CSS Secrets (1st ed.). O’Reilly Media. p. 392. ISBN 978-1-4493-7263-7.
PublicationsSmashing Book #3: Redesign The Web Smashing MediaMay 2012 Co-authored the CSS3 chapter of Smashing Book #3 with David Storey.Authors:Lea Verou, David Storey
CSS Secrets O’Reilly Media June 2015 Better solutions to everyday Web Design problems Authors: Lea Verou
- http://www.smashingmagazine.com/2011/10/ PrefixFree: Break Free From CSS Prefix HellBy Lea Verou Link: http://www.smashingmagazine.com/2011/10/prefixfree-break-free-from-css-prefix-hell/
- CSS Secrets Better Solutions to Everyday Web Design Problems By Lea Verou Publisher: O’Reilly Media Published June 2015
- Dabblet http://lea.verou.me/2011/12/introducing-dabblet-an-interactive-css-playground/
- Gilbertson, Scott (15 February 2013). “Interview: Lea Verou on Why Web Standards Matter and How You Can Help”. Wired magazine (Wired).
. Photo by Morgan Roderick http://www.flickr.com/photos/morganroderick/6982899366/in/set-72157629569953346|Original (used under the Creative Commons license)
. Photo by Jeffrey Zeldman https://www.flickr.com/photos/zeldman/9600870404 (used under the Creative Commons license)
. Lea Verou’s CV http://lea.verou.me/cv.html
.CSS Secrets Better Solutions to Everyday Web Design Problems By Lea Verou Publisher: O’Reilly Media Published June 2015