strange behaviour when using child theme, on slideout site template

Hello all,

I have a clean empty site, and I imported style and content from the 'slideout' demo.

Website looks as intended:

Now, first thing I do - before anything - is to install and activate a child theme.

I use the simple child theme provided here.

Now, strange thing is whenever the child theme is activated, I see a subtle change in the site:

the header, including menu, are no longer 'on the side' - but instead appear on top.

I 've added no code into child theme's css or php files. Cleared caches etc .. But problem remains and persists, i.e. de-activating child theme makes it look like intended again and vice-versa..

I guess it must be something really simple that I missed - I searched the forum but could not find an answer..

Would appreciate any help.