Updated light theme visited link colour to make text visible (#15406)

* Added css for consistent underline and colour on links

* Added css variable to light theme

* Add old CSS property as fallback

* Changed visited link colour to purple for light theme

* Changed visited link colour in dark/light themes for better contrast

* Update packages/theme-dark-extension/style/variables.css

Co-authored-by: Michał Krassowski <[email protected]>

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Co-authored-by: EC2 Default User <[email protected]>
Co-authored-by: Frédéric Collonval <[email protected]>
Co-authored-by: Michał Krassowski <[email protected]>

packages/theme-dark-extension/style/variables.css

@@ -167,7 +167,7 @@ all of MD as it is not optimized for dense, information rich UIs.
  --jp-content-font-color2: rgba(255, 255, 255, 0.7);
  --jp-content-font-color3: rgba(255, 255, 255, 0.5);
  --jp-content-link-color: var(--md-blue-300);
- --jp-content-link-visited-color: rgba(255, 255, 255, 0.3);
+ --jp-content-link-visited-color: var(--md-purple-300);
  --jp-content-link-hover-color: var(--md-blue-400);
  --jp-content-font-family: system-ui, -apple-system, blinkmacsystemfont,
  'Segoe UI', helvetica, arial, sans-serif, 'Apple Color Emoji',

packages/theme-light-extension/style/variables.css

@@ -165,7 +165,7 @@ all of MD as it is not optimized for dense, information rich UIs.
  --jp-content-font-color2: rgba(0, 0, 0, 0.54);
  --jp-content-font-color3: rgba(0, 0, 0, 0.38);
  --jp-content-link-color: var(--md-blue-900);
- --jp-content-link-visited-color: rgba(255, 255, 255, 0.3);
+ --jp-content-link-visited-color: var(--md-purple-700);
  --jp-content-font-family: system-ui, -apple-system, blinkmacsystemfont,
  'Segoe UI', helvetica, arial, sans-serif, 'Apple Color Emoji',
  'Segoe UI Emoji', 'Segoe UI Symbol';