i wanted to use the valo theme variable: $v-textfield-background-color–readonly in order to change the background-color of it.
Unfortunately i does not seem to get picked up. As another example i used $v-border-radius right next to it, which worked very well.
It seems that it is an issue in valo. If you just open _textfield.scss in your IDE you will see that $v-textfield-background-color–readonly variable does not have !default mark, so it is impossible to redefine this variable in a theme extension. We will fix it in our fork of Vaadin and I will file this issue to the Vaadin bug tracker.