Fix dark styles of buttons

[backrunner]

PR Checklist

• The commit message follows guidelines for NexT.
• Tests for the changes was maked (for bug fixes / features).
  • Muse | Mist have been tested.
  • Pisces | Gemini have been tested.
• Docs in NexT website have been added / updated (for features).

PR Type

• Bugfix.
• Feature.
• Code style update (formatting, local variables).
• Refactoring (no functional changes, no api changes).
• Build & CI related changes.
• Documentation content changes.
• Other... Please describe:

What is the current behavior?

Issue resolved: N/A

What is the new behavior?

In current released version, when dark mode and scheme Pisces or Gemini is enabled, the button in post-expand is still using the default styles:

I made a set of dark styles for these buttons:

hover:

• Link to demo site with this changes: N/A

[stevenjoezhang]

Is it possible to move all dark mode related changes into source/css/_colors.styl?
Thanks!

[backrunner]

Is it possible to move all dark mode related changes into source/css/_colors.styl?
Thanks!

Done, styles seems working fine, but more test may be required.