Make text darker and increase letter-spacing

[lukaw3d]

Fixes #1439 (comment)

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ CSS	stylelint	1	0	0	0.88s
✅ EDITORCONFIG	editorconfig-checker	21		0	0.03s
✅ REPOSITORY	checkov	yes		no	17.12s
✅ REPOSITORY	git_diff	yes		no	0.0s
✅ TSX	eslint	4	0	0	4.99s

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[codecov]

Codecov Report

Merging #1451 (3cd37e0) into master (f9b96e1) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

❗ Current head 3cd37e0 differs from pull request most recent head 6fdbacf. Consider uploading reports for the commit 6fdbacf to get more accurate results

@@            Coverage Diff             @@
##           master    #1451      +/-   ##
==========================================
- Coverage   82.99%   82.92%   -0.08%     
==========================================
  Files         154      154              
  Lines        4039     4040       +1     
  Branches      726      727       +1     
==========================================
- Hits         3352     3350       -2     
- Misses        687      690       +3     

Flag	Coverage Δ
cypress	50.91% <100.00%> (+0.01%)	⬆️
jest	78.05% <100.00%> (-0.10%)	⬇️

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Impacted Files	Coverage Δ
src/app/components/TransactionPreview/index.tsx	90.47% <ø> (-0.44%)	⬇️
src/styles/theme/ThemeProvider.tsx	100.00% <ø> (ø)
src/app/components/PrettyAddress/index.tsx	80.00% <100.00%> (+1.42%)	⬆️
...ponents/Toolbar/Features/AccountSelector/index.tsx	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

[buberdds]

lint