Provide custom themes/content for each tool category

Continuing with placeholder content, this demonstrates how we might style different categories of applications. With this approach, we can provide a set of themes that the user can choose between via layout.theme.theme = 'MyThemeName'.

This also provides a functional demonstration of how we can provide custom components in Trame that simplify building components that require a lot of nested boilerplate (such as VContainers).

Closes #21 (closed)