fix: Navbar styles and Welcome page text

[geido]

SUMMARY

It fixes an issue with the text in the Welcome page not appearing as reported here #19579. It also enhances the styles to follow our guidelines.

BEFORE

AFTER

DEV.Superset.mp4

TESTING INSTRUCTIONS

1. Go to the login page and make sure the logo appears in the right size immediately
2. Go to the Welcome page and make sure the text "Home" at the top is displaying correctly.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[etr2460]

a few comments

[etr2460]

thanks for the css improvements, lgtm!

[codecov]

Codecov Report

Merging #19586 (8ee288c) into master (7b0b029) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 8ee288c differs from pull request most recent head 392a9bb. Consider uploading reports for the commit 392a9bb to get more accurate results

@@            Coverage Diff             @@
##           master   #19586      +/-   ##
==========================================
- Coverage   66.47%   66.46%   -0.01%     
==========================================
  Files        1681     1681              
  Lines       64467    64448      -19     
  Branches     6607     6607              
==========================================
- Hits        42856    42837      -19     
  Misses      19917    19917              
  Partials     1694     1694              

Flag	Coverage Δ
javascript	51.12% <100.00%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...perset-frontend/src/views/CRUD/welcome/Welcome.tsx	74.22% <100.00%> (-0.78%)	⬇️
superset-frontend/src/views/components/Menu.tsx	50.00% <100.00%> (-10.00%)	⬇️

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