This is how BSN looks by default on Internet Explorer:

Meilleure réponse Jesse Reid , 25 février 2014 - 08:35
Windows Phone 8 should now by default use the mobile skin, I missed a checkbox.
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Guest_Rubios_*
How many people actually use Internet Explorer?
Guest_Rubios_*
There are literally dozens of us.
How many people actually use Internet Explorer?
IE10 and IE11 are reather nice from developer point of view. Nothing wrong in them. It's annoying that firefox for exmple still doesn't support unprefixed box-sizing property for example. On top of that the hardware acceleration is superior in IE11, makes Firefox feel really sluggish and inneficient. Also firefox lacks MSE support which would help when it comes to HTML5 videos (for that reason youTube HTML5 player only support up to 720p videos on firefox).
But anyway, OP: You can change the theme manually so it's not massive issue. I will stick but apparently it's account specific setting so by the looks of it it will change the theme on all browsers.