Support giscus themes

[vieitesss]

Description

I'm trying to set the giscus theme to Gruvbox Dark but it doesn't seem to be supported.

giscus:
    theme: gruvbox_dark

It would look amazing with the default dark theme.

[vieitesss]

I have found that I can just copy this file with the same path and change the theme.

<script>
  (function () {
    const themeMapper = Theme.getThemeMapper('dark', 'gruvbox_dark');
    const initTheme = themeMapper[Theme.visualState];