This PR puts the `Theme` returned from the `theme` function behind an
`Arc`.
### Motivation
While working on wiring up window focus events for the `TitleBar`
component we ran into issues where `theme` was holding an immutable
borrow to the `ViewContext` for the entirety of the `render` scope,
which prevented having mutable borrows in the same scope.
### Explanation
To avoid this, we can make `theme` return an `Arc<Theme>` to allow for
cheap clones and avoiding the issues with the borrow checker.
Release Notes:
- N/A
Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>
