It is possible to do it. And it probably happens naturally for a handrolled toolkit. However when the (one) developer of the GUI is different than the (many) app developers using it things typically end up different.
For instance you end up with things like ui designer tools that produce manifest files, where you just expect that some non-programmer can edit it and load it into your app without having to rebuild the toolkit.
For instance you end up with things like ui designer tools that produce manifest files, where you just expect that some non-programmer can edit it and load it into your app without having to rebuild the toolkit.