docs: ADR that proposes a common stylesheet for all MFEs

Adds an ADR that proposes creating a single common stylesheet that all MFEs load
from a central place.

docs/decisions/0007-compile-common-theme-for-all-mfes.rst

@@ -0,0 +1,139 @@
+0007 Compile Common Theme for all MFEs
+######################################
+
+
+Status
+******
+
+Draft
+
+Context
+*******
+
+Currently each Open edX MFE includes a small snippet of SCSS code that is common
+to all of them, which is the following four lines:
+
+.. code-block:: SCSS
+
+	@import "~@edx/brand/paragon/fonts";
+	@import "~@edx/brand/paragon/variables";
+	@import "~@edx/paragon/scss/core/core";
+	@import "~@edx/brand/paragon/overrides";
+
+These four lines, import the Paragon and bootstrap code and apply
+branding-specific overrides on top. They contain the bulk of the SCSS code
+generated for MFEs. Following this are usually imports for the frontend header
+and footer components, imports from the MFE own components and MFE-specific
+SCSS.
+
+This effectively means that all MFEs share a large chunk of common styling code
+with a comparatively much smaller set of styles applied over that.
+
+There is scope here to properly split the MFE stylesheet into multiple parts,
+one being the common code as represented by the above imports, and another for
+MFE-specific code, and build them separately.
+
+One thing that prevents this is that MFEs need these SCSS imports to get
+variables. In a number of cases these can be replaced with either
+Paragon/Bootstrap classes, or CSS variables. A separate ADR in `Paragon
+<https://github.com/openedx/paragon/pull/1388>`_ covers the situations where
+this isn't possible yet.
+
+Decision
+********
+
+Common cores styling that is to be applied to all MFEs should be part of a
+separate stylesheet and built separately from the MFE-specific stylesheet.
+
+Both these stylesheets will be included in each MFE, however the common
+stylesheet can be loaded from a common source for all MFEs. This will allow for
+deploying the common theme stylesheet without redeploying MFEs.
+
+The mechanism for loading this common stylesheet will need to be different from
+the mechanism for loading the MFE stylesheet. A JavaScript theme loader will
+contain the location of the SCSS file and will add this stylesheet to the
+document.
+
+The common theme package will be similar to the branding package, but can be
+deployed independently of MFEs. It will produce a CSS stylesheet, and a
+JavaScript loader that will load this stylesheet. When deploying MFEs, they can
+be provided a configuration variable that contains the location where this
+theme loader is deployed.
+
+The frontend-platform repo can host code that simplifies using themes. It can
+check if a theme loader is configured, and if so, dynamically load the theme
+from there. Otherwise, it can continue using the existing mechanism of building
+and deploying the theme for each MFE for backwards-compatibility. It can also
+provide hooks for loading the theme and potentially/eventually also selecting
+a theme.
+
+Consequences
+************
+
+With a single common stylesheet for all MFE, they should load quicker since the
+stylesheet can be cached once for all MFEs. It also makes theme rebuilds much
+quicker, and eliminates the need for rebuilding each MFE for minor change to
+the theme.
+
+For context, on a sample development machine, rebuilding an MFE takes roughly
+one minute, of this, on the same machine, around 3-4 seconds is the time it
+takes to build the theme.
+
+By continuing to support the existing theming mechanism, the current mechanism
+for theming can continue working for those who need it.
+
+Since there is a JavaScript-based loader involved, there is potential for
+flashing of unstyled content during loading, however this can be avoided by
+making the theme loader part of the init process so it runs before the app is
+considered ready.
+
+Another major consequence of this change would be that the CSS code for Paragon
+would be loaded using a different mechanism than the JavaScript code of Paragon.
+The CSS code would be compiled into the theme whereas the JS code will be loaded
+directly. This means that if different versions of Paragon are used for an MFE
+vs the common theme, then there is a chance that an incompatible version of the
+CSS could be loaded for the MFE. This will make it all the more important to
+ensure that version of Paragon used across all MFEs is compatible. This will be
+simpler for named releases than for those running latest code changes.
+
+Rejected Alternatives
+*********************
+
+- **Loading the common CSS directly instead of with a JavaScript based loader**
+
+  This ADR proposes using an intermediary JavaScript loader to load CSS rather
+  then just having the configuration variable point to the stylesheet itself.
+  This is done for performance reasons.
+
+  Currently, when an MFE is built, each static asset contains the hash of the
+  asset in the file name. The ``index.html`` file includes these generated file
+  names, and if any asset changes the ``index.html`` file is updated.
+
+  When a user reloads an MFE page the index page is reloaded, and if any assets
+  have changed, their file names will have as well and they will be loaded
+  fresh. Unchanged assets will keep using the same name and can load from the
+  cache for a very long time.
+
+  If the file names don't change with the content, then there is a risk of
+  loading older code from the cache, or (if caching is disabled) of loading a
+  huge chunk of code with each refresh and having a huge hit on performance.
+
+  So to maximise the amount of time for which stylesheets are cached, and
+  minimise the amount of code that needs to be loaded on each refresh, we need
+  to have a light intermediary loader that provides the location of the
+  stylesheet. The bulk of the CSS can then be cached for as long as it is
+  unchanged.
+
+- **Have the theme loading code in each MFE**
+
+  This doesn't change much about the rest of the mechanism of this ADR, but it
+  causes a lot of code duplication across MFEs. Just like ``frontend-platform``
+  handles authentication for all MFEs, it can also handle dealing with the theme
+  loader.
+
+
+References
+**********
+
+- `ADR to introduce CSS variables to parargon
+  <https://github.com/openedx/paragon/pull/1388>`_