# `Expresso.Theme`
[🔗](https://github.com/rellen/expresso/blob/main/lib/expresso/theme.ex#L1)

The custom properties, the effects and the transitions of the theme

`assets/style.css` is the theme of this project, and the theme owns the
custom properties. This module reads the file at compile time with
`Expresso.Css.scan/1`, and it gives four answers. `uses?/1` tells you
whether the theme uses a property. `declares?/1` tells you whether the theme
gives the property a value. `syntax/1` gives the `syntax` descriptor of the
`@property` rule of the theme. `effects/0` gives the effects of the theme,
and `transitions/0` gives its transitions between slides.

`Expresso.Overlay.Properties` reads the answers, and
`Expresso.Overlay.PropertyVerifier` then gives a warning for a property that
the theme does not use. The CSS of a deck adds its own names.
`docs/overlays.md` gives the contract.

# `declares?`

```elixir
@spec declares?(atom()) :: boolean()
```

Tell whether the theme gives the custom property of a name a value that is
not a number

The compiler registers each state as a number with the initial value 0. That
registration makes a value of a different type invalid, and the property then
falls back to 0. A number stays valid. Therefore a state must not take the
name of a property that the theme gives a value of a different type.

# `effects`

```elixir
@spec effects() :: MapSet.t(String.t())
```

Give the effects of the theme, with the fade

# `names`

```elixir
@spec names() :: Expresso.Css.names()
```

Give the names of the theme, for `Expresso.Css.merge/2`

The fade has no rule, because each element with steps fades. The function
adds it to the effects. The browser fades from one slide to the next
without a rule, and the kind `none` has no animation, so the function adds
the two to the transitions.

# `syntax`

```elixir
@spec syntax(atom()) :: String.t() | nil
```

Give the `syntax` descriptor of the `@property` rule of the theme

The function gives `nil` for a property that the theme does not register.

# `transitions`

```elixir
@spec transitions() :: MapSet.t(String.t())
```

Give the transitions of the theme, with the fade and the kind `none`

# `uses?`

```elixir
@spec uses?(atom()) :: boolean()
```

Tell whether the theme uses the custom property of a name

The name comes from the `state` option or from a key of the `set` option of
an `on` entity.

---

*Consult [api-reference.md](api-reference.md) for complete listing*