feat: add giscus theme customization

[ywonchae1]

To customize your giscus theme, select a theme from https://giscus.app/ and update the relevant option in the hugo.yaml file.

params:
  comments:
    giscus:
      theme: noborder_light    #  dark_high_contrast  |  dark_protanopia  |  cobalt  |  etc.

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Related issue #521

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