Vaadin Fusion Styling of Sub-Components

How do I apply part styling to a nested Vaadin component? Vaadin components expose "parts" for styling in their published APIs.

Specifically, the vaadin-upload component hosts another component, vaadin-upload-file. I can style the main vaadin-upload component alright, but how do I reach the parts of the nested vaadin-upload-file component?

E.g. for the "name" part of vaadin-upload-file I unsuccessfully tried CSS selectors like

[part="name"]  { ... // plain, as if it were passed through
vaadin-upload-file[part="name"]  { ... // qualified with the component name
[part="file-list"][part="name"] { ... // qualified by the part of vaadin-upload that hosts the file list
:host([part="file-list"]) [part="name"] { ... // same with :host() selector

This is all in the style of the component that deploys vaadin-upload.

Solution

As the answer made its way to me on a different path, here is the solution for SO:

You can only apply styles to Vaadin components and their sub-components using a custom theme.

Here is a slightly extended way to set this up using a parent theme.

Create a local custom theme in your app
- path is frontend/themes/<my-theme-name>/
- must contain a sub-directory components (to style Vaadin components)
- must contain a styles.css (even empty)
- must contain a theme.json with contents
```
{
   "parent": "<my-parent-theme>"
}
```
  but there are other keys for theme.json like importCss, documentCss or assets
- the parent theme can be a pom dependency

Use the custom theme in your Application.java:

@Theme(value = "<my-theme-name>")
public class Application extends SpringBootServletInitializer ...

Then you can add style files locally to style Vaadin components, e.g.
- add frontend/themes/<my-theme-name>/components/vaadin-upload.css:
```
[part=file-list]::before {
  content: "These are the current files:";
}
```
- frontend/themes/<my-theme-name>/components/vaadin-upload-file.css:
```
[part=name] {
  background-color: yellow;
}
```

This applies to Vaadin21 (e.g. in Vaadin19 using a parent theme is not working this way).