Hahaha I know it looks super-flat! But it’s not “more flat”. It’s just a recolor with a bucket tool of the green button. The green button consists exactly of two colors - green and white - and I changed those by colorpicking from the old button to blue and brown. 
And then tried to match font size and spacing and bordered the text. That’s all. The yellow button has a glow effect added to the edges, that takes literally ten seconds in Photoshop, which is why it looks different from the green one.
Also they cannot let us customize the UI; running two UI has already been out of the question as it would take coding for both of them (as explained by Lyya here: Serious question and conclusion on the New UI - #78 by Lyya). There’s no way customizing will ever be doable and this time it’s not a problem of the devs not wanting to do it but simply that it’s impossible, work-load wise.