Did you have a look at the JEM settings (Joomla! top menu: Components -> JEM -> Settings)?
Do you use the responsive layout style?
Before you change CSS, please have a full look at all settings. Quite a lot can be tweaked.
If you really want to change the CSS, there is a nice possibility in: Settings -> Layout: Custom Stylesheets.
I am pretty sure, you will not have to change the CSS to get to the demo result.
On my own website simplelist works like in the demo - and I did not change its CSS.
Hmmm, did you change to responsive layout resently and didn't you delete the cache?
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