Is there a windows fluent theme available for Vaadin?
I am not aware that there is one. Generic CSS kits wont work with Vaadin components. I know there are some metro/fluent look a like developments done on top of Bootstrap for example.
Would it be possible to style Vaadin components to fit metro/fluent. To some extent it is possible, it just requires some work. Some compromises is however needed, since you cannot change internal DOM structure of the web component by CSS. So for example if you take Vaadin’s DatePicker, you cannot change years browser to be in different place etc.
Would it be possible to create Java API for Metro/Fluent CSS Bootstrap kit. This is also to some extent possible and may be applicable for some applications. However it may not be interoperable with other Vaadin components. Usually 3rd party CSS kits have different z-index usage, which means that everything that uses overlays may be in different order and un-expected outcomes are possible. Also then you would miss sophisticated Vaadin components which have lazy loading features like Grid and ComboBox.