Describe how best to support light and dark themes when adding UI components

[lognaturel]

#2061 adds support for a dark theme. 🎉

As new UI components are added, they need to use colorControlNormal, primaryTextColor, etc to make sure the look changes based on the the theme. Advice on how best to do that should be added to https://github.com/opendatakit/collect/blob/master/CONTRIBUTING.md. This should also be linked to from the pull request template.

@shobhitagarwal1612, as the feature developer, I think you are best positioned to take a first pass when you get a chance! 😊

[shobhitagarwal1612]

Thanks @lognaturel
Will definitely do it soon 😄

[jd-alexander]

@shobhitagarwal1612 How does the buttons on the MainScreenActivity get their colour? I want to use that setup for a component but at first glance I haven't been able to figure it out.

[shobhitagarwal1612]

@jd-alexander the buttons are coloured implicitly. We don't need to mention the colour, it automatically picks from the current theme