docs: fix update theme property to a reactive property

[GmavionR]

Update theme to reactive property

Description of Problem

Theme should be a reactive property before you can use v-bind in css.

Proposed Solution

Additional Information

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[AloisSeckar]

Hello, and thank you for your interest in improving Vue.js docs. But your assumption is wrong. Static values can be bound easilly - see live demo here.

Using reactive would become useful to allow dynamic runtime changes of the bound color. But this is not what this simple demo is focusing at, so I think it would be rather confusing than helpful to include it here.

[brc-dd]

I think having reactive will be more consistent with the options API example above it, and the description below that states that the styles are reactively updated if the source value changes. We can probably make the example more useful than just keeping it simple because I don't think any one would use CSS data binding for a static value.

PS: remove extra ref import

[bencodezen]

Great catch @GmavionR! I modified the example you submitted down to only import ref and use that instead of reactive since this is the one we'd like to see the community standardize on if possible.

Thank you for the PR!