Centered form with material design lite

How about using “mdl-layout-spacer”: See codepen

<div class="mdl-grid">
    <div class="mdl-layout-spacer"></div>
    <div class="mdl-cell mdl-cell--4-col">This div is centered</div>
    <div class="mdl-layout-spacer"></div>
</div>

Or if you prefer a css solution:
Add an extra class to the grid containing the column to be centered. See codepen

<div class="mdl-grid center-items">
    <div class="mdl-cell mdl-cell--4-col">This div is centered</div>
</div>

.mdl-grid.center-items {
  justify-content: center;
}

Both options play nice when resizing.