Add custom css selectors to elements
I would love the ability to be able to name elements with that name being applied to the presentation element (including the slide background) as a css class selector.
The purpose for this being to apply individual transitions & animations with CSS to specific elements only, for instance I may which to cycle the slide background color through a range of colors.
To save on system resources this would only apply to the present slide e.g.
.present.customSelector {
/* Apply custom css */
}

This is now supported! Turn on the developer mode and you’ll have access to easy inputs for specifying CSS classes.
Classes can be added to individual elements within a slide or to the slide itself. Classes added to a slide also bubble up to its background element.
More info here: http://help.slides.com/knowledgebase/articles/454562-developer-mode
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AdminOwen (Owner, Slides) commented
We're all for this idea and this is actually something we already built into the teams product, as part of the theme editor. Though it's very hidden for the time being.
The way it works is you define a custom class using special CSS syntax [1] and those classes are then exposed through the editor UI [2]. When one of these classes are added to a slide it will also apply to the corresponding background element.
[1] http://cl.ly/image/1s432c1t461W
[2] http://cl.ly/image/3s2w141t3k1dWe'd eventually like to make the theme editor available to all Pro users, but we haven't started working towards that yet.
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Richard Duncan commented
I've realised that you can edit the majority of elements HTML and add a class or ID selector in there but I would still like some ability to configure the background.