Vertically align forms and Css

<div class="list">
  <label class="item item-input">
    <span class="input-label">Username</span>
    <input type="text">
  <label class="item item-input">
    <span class="input-label">Password</span>
    <input type="password">
  <button class="button button-full button-positive">

how to align this kind of form in the vertically center this kind of form?

almost always when i try put in some css the sass simples compile over my command and make it useless
so how do i know it’s safe to alter the css with less if i don’t have multiple cellphone for test?

for my case i guess something as simples as

<style="position: absolute; top: 50%;">

should get the job done