[chore] fix stylelint warnings
```sh
[build] [15:42:11] Starting 'scss'...
[build]
[build] assets/css/_custom.scss
[build] 12:3	⚠  Unexpected unknown property "font-smoothing" (property-no-unknown) [stylelint]
[build]
[build] [15:42:12] gulp-postcss: css/_custom.scss
[build] stylelint: /Users/jack/Projects/lad.sh/assets/css/_custom.scss:12:3: Unexpected unknown property "font-smoothing" (property-no-unknown)
[build] [15:42:12] gulp-imagemin: Minified 9 images (saved 104 kB - 35.3%)
[build]
[build] assets/css/_markdown.scss
[build] 24:3	⚠  Expected selector ".markdown-body code" to come before selector ".markdown-body pre > code" (no-descending-specificity) [stylelint]
[build] 27:14	⚠  Expected selector ".markdown-body pre" to come before selector ".markdown-body .highlight pre" (no-descending-specificity) [stylelint]
[build] 30:3	⚠  Expected selector ".markdown-body pre" to come before selector ".markdown-body .highlight pre" (no-descending-specificity) [stylelint]
[build] 30:3	⚠  Unexpected duplicate selector ".markdown-body pre, .markdown-body .highlight pre", first used at line 27 (no-duplicate-selectors) [stylelint]
[build]
[build] [15:42:12] gulp-postcss: css/_markdown.scss
[build] stylelint: /Users/jack/Projects/lad.sh/assets/css/_markdown.scss:24:3: Expected selector ".markdown-body code" to come before selector ".markdown-body pre > code" (no-descending-specificity)
[build] stylelint: /Users/jack/Projects/lad.sh/assets/css/_markdown.scss:27:14: Expected selector ".markdown-body pre" to come before selector ".markdown-body .highlight pre" (no-descending-specificity)
[build] stylelint: /Users/jack/Projects/lad.sh/assets/css/_markdown.scss:30:3: Expected selector ".markdown-body pre" to come before selector ".markdown-body .highlight pre" (no-descending-specificity)
[build] stylelint: /Users/jack/Projects/lad.sh/assets/css/_markdown.scss:30:3: Unexpected duplicate selector ".markdown-body pre, .markdown-body .highlight pre", first used at line 27 (no-duplicate-selectors)
[build] [15:42:12] Finished 'img' after 1.81 s
[build]
[build] assets/css/app.scss
[build] 25:16	⚠  Unexpected missing generic font family (font-family-no-missing-generic-family-keyword) [stylelint]
[build] 30:16	⚠  Unexpected missing generic font family (font-family-no-missing-generic-family-keyword) [stylelint]
[build]
[build] [15:42:12] gulp-postcss: css/app.scss
[build] stylelint: /Users/jack/Projects/lad.sh/assets/css/app.scss:25:16: Unexpected missing generic font family (font-family-no-missing-generic-family-keyword)
[build] stylelint: /Users/jack/Projects/lad.sh/assets/css/app.scss:30:16: Unexpected missing generic font family (font-family-no-missing-generic-family-keyword)
[build] [15:42:12] Finished 'scss' after 423 ms
```