Phiki comes with a set of built-in Visual Studio Code themes, but you can also create and use your own custom theme.
To learn more about writing custom Visual Studio Code themes, check out the Visual Studio Code documentation on theming.

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Local files

If your theme is stored in a local JSON file, you can load it by passing the slug of the theme and file path to the theme() method. 
use Phiki\Phiki;

$phiki = (new Phiki)
    ->theme('my-theme', '/path/to/my-theme.json');

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Runtime themes

You can also define themes at runtime by providing a theme definition as an associative array to the Theme::parse() method. 
use Phiki\Phiki;
use Phiki\Theme\Theme;

$phiki = (new Phiki)
    ->theme('my-theme', Theme::parse([
        'name' => 'my-theme',
        'colors' => [
            // ...
        ],
        'tokenColors' => [
            // ...
        ]
    ]));

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Usage

To use your custom theme, simply pass the slug you defined when registering the theme instead of a Phiki\Theme\Theme enum value. 
$html = (new Phiki)
    ->theme('my-theme', '/path/to/my-theme.json')
    ->codeToHtml('Your code here...', Grammar::Php, 'my-theme')
    ->toString();