Inbuilt theme creator?

We've got several themes, but [creating a new theme is difficult](https://steemit.com/bitshares/@cm-steem/bitshares-ui-customization-guide) and requires technical skills potentially outwith of theme creators.

If there was a theme creator tab in the settings where you could create and apply a custom theme by inputting all theme related CSS values (optionally using a colour palette), and enable sharing (export/import css) somehow then we may end up with hundreds of custom theme variations created.

Theme creator themes wouldn't be shared to all web wallets, we should rely on exporting the theme CSS for inclusion [alongside official themes in github](https://github.com/bitshares/bitshares-ui/tree/staging/app/assets/stylesheets/themes).

Perhaps an addition to the current theme dropdown could be 'customize', and the CSS configuration details appear for editing/exporting.

Thoughts?

Review app/assets/stylesheets/themes and the existing theme dropdown and settings entry points. Determine how official themes are represented and what requirements are needed for editing, applying, exporting, importing, and sharing custom CSS. Done should be an agreed scope and acceptance criteria for the proposed theme creator.

Written by the indexing model from the issue text.