custom.css : diferent behaviour with development mode ON or OFF

Hi there,

I'm currently trying to figure out why my site looks different when development mode is set ON or OFF. I did start from a T3_blank, set template development mode ON, and then customized the following elements :

- Layout (php files in tpls/)
- blocks
- variables.less (in less/) (only to adjust width and gutters)
- custom.css (in css/)

Everything works fine untill I set development mode OFF : then the pages seem broken (blocks size, hidden positions, ...). It works again when I turn dev mode OFF, but I would like to understand what I did wrong, and gain performance with dev mode OFF.

I started the site with the previous version of T3, that i updated to the newest version...

Thanks for your precious help

Eric