p.s. I'm also seeing that subsequent page load from ?theme=fastr-child occasionally loads the Twenty Twelve theme. While /category is correctly loading fastr-child. At this stage, I have three 'Sticky' keyword=value defined for three themes. Some sort of issue with how the keyword=value string is overriding one another?...
I open more than one tabs in my browser when testing, and occasionally using different keyword=value pair for each tab.
So, another test with history deleted (everything) in Firefox...
First, load http://theastro.staging.wpengine.com/?theme=fastr-child
Second, click 'Cancer' in the June 27th article, which takes me to
http://theastro.staging.wpengine.com/astrology-glossary/cancer-622-722/
...with Twenty Twelve theme
Third, click 'Back' in browser, return to homepage with fastr-child theme, then click June 27th article (title), this time, properly taken to http://theastro.staging.wpengine.com/the-astrologer-daily-june-27th-2014/ with fastr-child theme
Fourth, click 'Cancer' again there, and taken to a situation similar to second step above.
Fifth, press Ctrl+F5, nothing changes.
Sixth, click "What’s Your Study Style, Cancer ?" on the 'Cancer' page above... then taken to http://theastro.staging.wpengine.com/whats-your-study-style-cancer/ with fastr-child theme loaded. At this stage, we went from homepage with fastr-child, a post with fastr-child, then a /astrology-glossary page with a Twenty Twelve theme, and then a post with fastr-child theme. So, in a continuous page to page movement, there's a non-intended theme appearing in between.
Seventh, copy paste http://theastro.staging.wpengine.com/ into browser address bar, and it shows homepage with Twenty Twelve theme.
Eight, load http://theastro.staging.wpengine.com/?theme=fastr-child and it shows properly fastr-child theme.
Ninth, load http://theastro.staging.wpengine.com/ and it again shows Twenty Twelve theme.
All of the above steps are done within a single browser tab.
Hope this info might help you troubleshoot?
Thanks again!... and let me know if you need access to the admin side. I can send login info to your email.