fix: linkedin icon color doesn't exist when user use linkedin in share.yml

[nshidqi]

Description

When users want to use Linkedin and Weibo for sharing by uncommenting them in share.yml, the LinkedIn icon doesn't have its color, whereas Weibo has its color. Turned out that Linkedin's icon color is missing in post.scss.

Solution:
Add Linkedin color in post.scss, the color is based on LinkedIn's official brand color:
https://brand.linkedin.com/content/brand/global/en_us/index/visual-identity/color-palettes

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How has this been tested

• I have run bash ./tools/deploy.sh --dry-run (at the root of the project) locally and passed
• I have tested this feature in the browser

Test Configuration

• Browser type & version: Google Chrome Version 104.0.5112.102 (Official Build) (64-bit)
• Operating system: Windows 11
• Ruby version: ruby 3.1.2p20
• Bundler version: Bundler version 2.3.21
• Jekyll version: jekyll (4.2.2)

Checklist

• I have performed a self-review of my code
• I have commented on my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

[cotes2020]

Thanks for your contribution!