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Fashion rules exist purely to be broken, right? Certainly, that seems to be Sienna Miller's approach, and we're here for her rebellious sartorial spirit.

The reigning style icon delivered yet another quality outfit on Monday when she stepped out in London alongside her boyfriend Oli Green after watching the opening night of Cabaret the Musical at the Kit Kat Club. 

Sienna looked characteristically elegant in a leather trench, styled alongside a pair of belted high-waisted trousers.

Sienna Miler walking with her boyfriend Oli Green © Getty
The actress was accompanied by her boyfriend Oli Green

The 42-year-old, who is just a few months postpartum, took the opportunity to flout an age-old colour rule in her outfit, and the result was seriously chic.

Sienna's glossy trench, by minimalist luxury label Frame, was black whereas her trousers, from Celine's 2016 Resort collection, were coloured in an elegant French navy.

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Sienna Miller wearing navy trousers and a black trench © Getty
Sienna wore an unexpected colour combination

Her colour combination evoked the, somewhat trite, unwritten fashion rule, discouraging the pairing of navy and black.

Like many style connoisseurs, Sienna paid no notice, instead leaning on instinct.

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Sienna Miller wearing a glossy trench © Getty
Sienna wore a glossy trench for the occasion

 The fluid silhouette of her trousers felt relaxed, but the layering of the glossy black trench propelled her outfit into dressier territory, presenting a masterclass in after-dark darks.

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 Sienna completed her ensemble with pieces from some of her most-worn designer labels, a croissant-shaped mini bag by Chloé, and pointed Prada mules.

 The finishing touches to her date night 'fit were a swipe of crimson across her lips and an unfussy updo – a beauty hallmark of understated glamour.