feat: improve subgraphs demo design

[tloffler]

Closes #395

Description:

This PR enhances the design of the Subgraphs demo in light and dark mode.
Changes:

• Updated the layout
• Adjusted typography and spacing
• Adjust colors

Steps:

Open the subgraphs demo example and check the design

Type of change:

• New feature
• Enhancement

How Has This Been Tested?

• Manual testing

Remember to check that:

• Your code follows the style guidelines of this project
• You have performed a self-review of your code
• You have commented your code in hard-to-understand areas
• You have made corresponding changes to the documentation
• Your changes generate no new warnings

Screenshots

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Name	Status	Preview	Comments	Updated (UTC)
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