Digging on the Valo Demo Source code, find that setting a number of values you can change the Scheme or theme for Valo, you can check how they manage there style for Metro on: https://github.com/vaadin/valo-demo/blob/master/src/main/webapp/VAADIN/themes/tests-valo-metro/_variables.scss, if needed thet also have the other themes: https://github.com/vaadin/valo-demo/tree/master/src/main/webapp/VAADIN/themes
This are the variables that they use to change the scheme to 'Metro':
$v-app-loading-text: "Metro Valo";
$v-font-family: "Source Sans Pro", sans-serif;
$v-app-background-color: #fff;
$v-background-color: #eee;
$v-focus-color: #0072C6;
$v-focus-style: 0 0 0 1px $v-focus-color;
$valo-menu-background-color: darken($v-focus-color, 10%);
$v-border: 0 solid v-shade;
$v-border-radius: 0px;
$v-bevel: false;
$v-gradient: false;
$v-shadow: false;
$v-textfield-bevel: false;
$v-textfield-shadow: false;
$v-textfield-border: 1px solid v-shade;
$v-link-text-decoration: none;
$v-overlay-shadow: 0 0 0 2px #000;
$v-overlay-border-width: 2px; // For IE8
$v-window-shadow: $v-overlay-shadow;
$v-selection-overlay-background-color: #fff;
$v-selection-overlay-padding-horizontal: 0;
$v-selection-overlay-padding-vertical: 6px;
$v-panel-border: 2px solid v-shade;
Just import or include this on your theme file.