Update DEX with custom frontend theme

[xunholy]

Details

DEX is installed using pre-existing themes that are available, namely coreos and tectonic.

source: https://github.com/dexidp/dex/tree/master/web/themes

To use a custom theme you need just to put it along with others into a web directory. The folder structure should be the same: logo, favicon, and CSS.

After that, add

frontend:
   theme: yourthemefolder

to your dex config.

This will need to be added as a volume mount using the DEX chart https://github.com/helm/charts/blob/05f1535dbe89bdb888f5a6b8b8b2cef57c9c672f/stable/dex/values.yaml#L116

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