Hi Elena and John,
First up in regards to the unrelated issue, it is most certainly a bug.
I've filed
MDL-22353 to track it and have assigned to Petr, fingers crossed we can implement a solution shortly.
As for the modern / old browsers question, the feature was added in recognition that older browsers are starting to be dropped from support lists of major websites and that there will more than likely be theme's developed within the immediate life-cycle of Moodle 2 that don't support these browsers.
Currently on the old browsers list are IE6 and IE7 I believe, two browsers that are renowned for giving web designers and developers headaches.
How it all works within Moodle?
You can now select two theme's, one that will be used for old browsers (IE6/7) and one that will be used for all other browsers.
If a user is using IE6/7 then they will see the theme you selected for old browsers, if they are using any other browser they will see the theme you selected for modern browsers.
When should it be used?
If you know the theme you are using is supported by all browsers then there is absolutely no harm is setting both modern and old browsers to use that theme.
If you know know of or suspect that the theme you want to use is problematic on those old browsers then it is probably a good idea that you select another theme for old browsers. This will help ensure that your users don't have a compromised experience.
Those of course are both guides, it is up to you how you make use of it all
Hope this helps and answers a few questions.
Cheers
Sam
