fix layout beautiful when browser changes size !

[zhuzhuyule]

PR Checklist

Please check if your PR fulfills the following requirements:

• [ ] The commit message follows our guidelines.
• [x] Tests for the changes have been added (for bug fixes / features).
  • [x] Mist have been tested.
  • [ ] Muse | Mist have been tested.
  • [ ] Pisces | Gemini have been tested.
• [ ] Docs have been added / updated (for bug fixes / features).

PR Type

What kind of change does this PR introduce?

• [x] Bugfix.
• [x] Code style update (formatting, local variables).
• [ ] Feature.
• [ ] Refactoring (no functional changes, no api changes).
• [ ] Build related changes.
• [ ] CI related changes.
• [ ] Documentation content changes.

What is the new behavior?

Description about this pull, in several words...

• Screens with this changes: you can resize brower, when brower size between 767px and $content-desktop(or $content-desktop-large) , the layout is terrible and the sidebar not hide.

• Link to demo site with this changes: zhuzhuyule.com