Currently, monochromatic themes are only installed if the user is on Nightly and they've flipped the browser.theme.temporary.monochromatic.enabled. We need to remove both these requirements ahead of a wide release.

In order to do so, we need to do the following:

• Stop uninstalling the themes on shutdown
• At startup, check the currently-applied theme. That theme can be installed in BrowserGlue._beforeUIStartup. The other monochromatic themes should be installed after UI startup. mixedpuppy suggested installing them in BrowserGlue._onFirstWindowLoaded, but he also suggested double-checking that.

This creates a centralized place to manage built in themes. The main motivation for this is that we want to defer intalling built-in themes until after UI startup, but we need to ensure those themes are installed when about:addons is visited. This module is called from both BrowserGlue (for installation) and aboutaddons.js (to ensure installation). Also, built in themes handing is all over the place right now, and this centralizes it, which is nice. Ed, I'm adding you an as optional reviewer since about:welcome will have to call BuiltInThemes.ensureBuiltInBrowserThemes() if we go this route.

This also creates a more-permanent pref for colorway themes. The rationale is that I could see Product wanting a ripcord on this project and I want to build that in from the start.

This approach has serious performance improvements over the baseline. Here is perfherder for the current approach of installing before UI startup: https://treeherder.mozilla.org/perfherder/compare?originalProject=try&originalRevision=82baacf0b728add88c7e262ecb68d3fbcd1bda5e&newProject=try&newRevision=169edd7e48e5a051e5d3cbfbd6caaab4c496cd98&framework=1&page=1&showOnlyImportant=1 and again for an early version of the approach in this patch (installation after UI startup, but before I moved things to LightweightThemeConsumer): https://treeherder.mozilla.org/perfherder/compare?originalProject=try&originalRevision=82baacf0b728add88c7e262ecb68d3fbcd1bda5e&newProject=try&newRevision=6c200f35430a6af87401bd21c8db95e28a568964&framework=1&page=1&showOnlyImportant=1.

https://hg.mozilla.org/mozilla-central/rev/8467b9367b5d