Add themes to templates

We should consider adding a `Theme` struct and pre-defined colors to the intermediate level templates, see [this commit](https://github.com/ratatui/templates/commit/36ecf5d4be66865fbf3630d3aff07e54e44ba29e#diff-4171f7d1bca629cb013981de49d84761fa1648ba681f45e7aad266a3b0422c25R60) for an example.