Override theme CSS by file

[yhatt]

This PR will support to override theme CSS at --theme option by file path.

--theme option may allow passing CSS file path in addition to the name of theme. The passed theme CSS is always used in all convertions even though theme was specified in Marp Markdown.

The notable point is that allows anonymous theme. Normally Marpit framework requires @theme metadata comment to theme CSS. But marp-cli will generate the unique name (like /* @theme ym2qyqimipqn2gfzjddbnrdrwrkb2vtkk3sjkdj0x */) for the theme CSS passed to --theme. It is useful when developing/debugging theme.

@import 'default';

section {
  background: #f93;
}

marp --theme theme.css slide.md

ToDo

• Add tests for --theme option by file