Revert "merge smcmahon's changes to custom.css and override the default styles this way. allow again pulling datepickers default styles by the compiler script'"

This reverts commit 62d66d4687120329558aa8f8a2ea57b72b218206.
Revert "- Create dedicated custom.css resource for Plone-integration customizations."

This reverts commit 98f745bd32070ca071a3eeb5a3cceb4f188551d2.
Re-add next/prev images, cacheability of dateinput.css