Improve documentation on extending/overriding the existing SvelteUI theme

[BeeMargarida]

Improvement

Add additional documentation and examples showing how it is possible to override the existing SvelteUI themes.

See:

• https://github.com/svelteuidev/svelteui/discussions/309
• https://discord.com/channels/954790377754337280/1096717959822590003/1096717959822590003

Possible implementation - describe how the feature can be implemented

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Do you want to contribute this feature and create a pull request

Yes