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

The Spark verifier of the contrast of the `theme` option

It runs one time for each deck module at compile time. A built-in theme
meets each minimum of `Expresso.Palette`, so only a theme of 16 colors can
fail. The verifier gives a warning for each role that fails, and the deck
still compiles. The option itself refuses a value that is not a theme.

The warning of a role names the slot of its color, and a color for that
slot that passes, from `Expresso.Palette.suggestions/1`. The colors of the
map stay as the user gives them.

The verifier checks each variant of the option that is a map, after the
adjustment of `adjust: true`, and the warning of a variant names it. An
adjustment that changes a color by more than the limit of a built-in theme
gives a warning too.

The verifier also gives a warning for each variant that does not dim at the
`dim_contrast` option of the deck. `dim_messages/2` gives the rules.

# `dim_messages`

```elixir
@spec dim_messages(term(), number() | nil) :: [String.t()]
```

Return the warning of each variant of a theme that does not dim at a minimum contrast ratio

`ratio` is the `dim_contrast` option of the deck, or `nil` for the default
of 3:1. A variant does not dim when its dim opacity is 1.0: then no opacity
under 1.0 keeps each role of text at `ratio` on its background. A dimmed
image, SVG file, embed and video then do not dim, because the style sheet
dims them with `--dim-opacity`. The warning also names each role of text
that keeps its color in the state `dim`.

A variant that fails a minimum of `messages/1` gets no warning here,
because its warnings of contrast tell the cause.

    iex> Expresso.ThemeVerifier.dim_messages(:default, nil)
    []
    iex> [message] = Expresso.ThemeVerifier.dim_messages(:solarized_light, 4.5)
    iex> message =~ "no role of text dims"
    true

# `messages`

```elixir
@spec messages(term()) :: [String.t()]
```

Return the warning of each role of a theme under its minimum of contrast

`theme` is a value of the `theme` option. `verify/1` and `Expresso.Check`
call this function.

    iex> Expresso.ThemeVerifier.messages(:dracula)
    []

# `verify`

```elixir
@spec verify(map()) :: :ok | {:warn, [String.t()]}
```

Give a warning for each role of the theme of the deck under its minimum of contrast

---

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