Add css var for indeterminate_cycle_duration

[lena]

Enable customization of the indeterminate duration cycle

Demo with 20s duration:

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[lena]

I signed it!

[googlebot]

CLAs look good, thanks!

[keanulee]

@lena This looks great! Mind adding a demo to the demo page?

[lena]

@keanulee sure thing 👍 will do it today

[lena]

@keanulee done

[keanulee]

LGTM. Thanks @lena!