Conditional for "reponsive"

[Mixy]

Hi there, I'm trying to make some modifications for mobile users and one thing I want is to set collapsed by default the categories on my index.

For this I need to change in forumbit_depth1_cat

<tbody style="{$expdisplay}" id="cat_{$forum['fid']}_e">

to

<tbody style= "display: none" id="cat_{$forum['fid']}_e">

But I'm not quire sure for the 'responsive' conditional. I've tried:

Code:

<if media=(max-width: 420px) then> <tbody style= "display: none" id="cat_{$forum['fid']}_e"> <else> <tbody style="{$expdisplay}" id="cat_{$forum['fid']}_e"> </if>

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And some variations with "" and the @media but they don't work. If someone could guide me, I would be very grateful. Thank you.