Implement scss variable injection by OC_Defaults

Signed-off-by: Julius Haertl <jus@bitgrid.net>

Add Scss variables to example theme and theming app

Signed-off-by: Julius Haertl <jus@bitgrid.net>

Use SCSSCacher to build theming css

Signed-off-by: Julius Härtl <jus@bitgrid.net>

Update theming.scss

Signed-off-by: Julius Härtl <jus@bitgrid.net>

Code cleanup

Signed-off-by: Julius Härtl <jus@bitgrid.net>

Fix tests

Signed-off-by: Julius Härtl <jus@bitgrid.net>

Inject SCSSCacher for easier testing

Signed-off-by: Julius Härtl <jus@bitgrid.net>

Fix typehint

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>

Generate absolute URLs

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>

Fix tests to always use absolute urls for theming images

Signed-off-by: Julius Härtl <jus@bitgrid.net>

MailheaderColor -> ColorPrimary

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>