The ExtJS 6.5 mobile themes certainly help to write single web apps that can be used on desktops, tablets or smartphones.
But you are limited to only 2-3 themes available that not everyone likes.
Having the possibility to change colors / fonts would be a further help but at the moment we have to do everything via JS and know exactly what to declare to acces EXTJS api.
However, a nice step forward compared to before ...