There are many different ways of accomplishing the same result that it looks like you’re aiming at, which is the ability for a user to choose a “personal” look and feel for your site. That’s a great feature, especially for a site which would discuss css styling.
However, personally, I would focus on writing valid standards compliant code (which also eliminates most cross browser issues) before trying to get fancy with the style switching (there are a number of plugins available which are capable of producing standards compliant code as well, which is a nice bonus for that type of effect).
Here are the w3c validator results for your existing xhtml and css files:
https://validator.w3.org/check?uri=http%3A%2F%2Fdearauthor.com%2Ftech%2F%3Fp%3D4
https://jigsaw.w3.org/css-validator/validator?uri=http%3A%2F%2Fdearauthor.com%2Ftech%2Fwp-content%2Fthemes%2Ffoliage_mod2%2Fstyle.css&usermedium=all
https://jigsaw.w3.org/css-validator/validator?uri=http%3A%2F%2Fdearauthor.com%2Ftech%2Fwp-content%2Fthemes%2Ffoliage_mod2%2Fstyle175.css&usermedium=all
https://jigsaw.w3.org/css-validator/validator?uri=http%3A%2F%2Fdearauthor.com%2Ftech%2Fwp-content%2Fplugins%2Faf-extended-live-archive%2Fincludes%2Faf-ela-style.css&usermedium=all
for me at least, a good part of the reason for using xhtml/css to begin with is to make my life easier by producing standards compliant, accessible, cross browser compliant code, thus eliminating any extra effort required to do such things as “tailor” separate style sheets for each browser, then attempt to make that system work with javascript which is not likely to work with all of the browsers the “special” css is written for anyway.
standards, standards, standards.
