I went through all the issues associated with adding a ?keyword=value string to the end of every link, and I just don't think it would be feasible to find all links:
- in every part of a WordPress "web page"
- without accidentally appending to non-WordPress links
I just don't think it is the best solution for you and anyone else with a similar kind of issue. Again, just my opinion.
What I am willing to explore is providing a Function that you can call from php that Sets the Theme using a Cookie that would reside in the browser cache of the visitor's computer. And have a Setting that tells my plugin to check for a Cookie with the Theme Name in it, previously stored via that Function. Something like jr_mt_set_theme('TwentyTen');
The end result would be a single call to that Function would then see all (for some period of time, measured in days) subsequent WordPress web pages to that Visitor use that Theme.
Would that solve your problem?