You are almost doing right.
Just replace below line in Form.ui.xml
file.
<g:Label res:styleName="style.text">ABC</g:Label>
with
<g:Label res:styleName="{res.style.text}">ABC</g:Label>
screenshot:
Problem : "style.text"
is not referring to your style defined in FormResources#MyCSS#text
.
Always enclose the value in {...}
if you want to use some dynamic values.
For more info have a look at sample on GWT UiBinder - Hello Stylish World