Align problem with ion-tabs

I have problem to resize the tabs. I don’t want it fullfill my screen, so i changed the _tabs.css:
/default value was 100%/
width: 50%;
This works, but it didn’t align center as i needed. So i add this to fix it:
margin:0 auto;
However, it didn’t work. Anyone got this problem before?

this is part of the scss file
width: 50%;
height: $tabs-height;

border-style: solid;
border-top-width: 1px;

background-size: 0;
line-height: $tabs-height;

@media (min–moz-device-pixel-ratio: 1.5),
(-webkit-min-device-pixel-ratio: 1.5),
(min-device-pixel-ratio: 1.5),
(min-resolution: 144dpi),
(min-resolution: 1.5dppx) {
padding-top: 2px;
border-top: none !important;
border-bottom: none;
background-position: top;
background-size: 100% 1px;
background-repeat: no-repeat;
margin:0 auto;

problem solved
actually, we shoulw put margin:0 auto outside the @media part.