margin left/right is zero when home: true in default theme

[gcnyin]

Describe the bug

margin left/right is zero when home: true

Reproduction

1. start a new vitepress project
2. in docs/index, set home: true
3. open in mobile

Expected behavior

should have left and right margin

System Info

System:
    OS: macOS 11.5
    CPU: (12) x64 Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
    Memory: 293.35 MB / 16.00 GB
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 12.22.1 - ~/.nvm/versions/node/v12.22.1/bin/node
    Yarn: 1.22.10 - ~/.nvm/versions/node/v12.22.1/bin/yarn
    npm: 7.17.0 - ~/.nvm/versions/node/v12.22.1/bin/npm
  Browsers:
    Chrome: 92.0.4515.107
    Firefox: 84.0
    Safari: 14.1.2
  npmPackages:
    vitepress: ^0.15.6 => 0.15.6

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.