Slide size defined in :root selector does not reflect to the theme instance

[yhatt]

Marpit document is explaining how to define slide size in :root selector through width and height declarations, but the generated Theme instance won't reflect the slide size.

Marpit.themeSet.add(`
/* @theme abc */
:root {
width: 123px;
height: 456px;
}
`)
// Theme {
//   name: 'abc',
//   css: '\n/* @theme abc */\n:root {\nwidth: 123px;\nheight: 456px;\n}\n',
//   meta: { theme: 'abc' },
//   importRules: [],
//   width: undefined,
//   height: undefined
// }

width and height must reflect the size defined in :root selector.

:root selector support in v1.6.0 was only added to themeSet.pack() to transform theme CSS into slide-specific style. Probably we have to alias while registering theme instead of while packaging.