+++ This bug was initially created as a clone of Bug #1659444 +++

This is a metabug for a project to stop using the preprocessor for our CSS. This will enable the use of tooling/linting on more of our CSS, which will help with keeping a consistent code style and making sure our CSS is accessible/understandable for new folks.

Roughly, it involves the following steps:

1. Replace our use of preprocessor variables (%define) with CSS variables.
2. Replace conditionals with other constructs (removing obsolete things, or changing to comments, variables, selectors or commented out preprocessor conditionals for UA sheets)
3. Package shared theme files into a shared/ subdirectory of the skin (theme) package, instead of packaging them into the same directory as the OS-specific ones.
4. Replace includes by either unifying files so we just have 1 shared file, inlining them, or using @import from a shared location.

I think we should be ready now.

We kind of are. There is still the semi shared panelUI.css but Firefox didn't clean up their copy yet (looks like low hanging fruit though)
https://searchfox.org/comm-central/search?q=panelUI.inc.css&path=

customizeMode.inc.css seems to be the last thing needed to finish this bug, so I'm adding the patch here. We can just remove it as we don't use it anywhere

Comment on attachment 9182578 [details] [diff] [review]
bug1661676_remove_css_preprocessing.patch

The other files, density-compact.svg, density-normal.svg, density-touch.svg, empty-overflow-panel.png, empty-overflow-panel@2x.png and whimsy.png are also not used. We could delete them too.

Pushed by mkmelin@iki.fi:
https://hg.mozilla.org/comm-central/rev/a06510a40d98
remove unused customizeMode.inc.css along with unused images. r=Paenglab

I left whimsy.png since it's kind of referenced.