MD WebUI: Move some settings CSS to shared_style_css
This CL makes the separator line and hover color shared for MD.
It also shares the [actionable] styling.
BUG=
521040
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation
Committed:
https://crrev.com/3ec487498ee6035b89e02b07cf03b7725de2ccc3
Cr-Commit-Position: refs/heads/master@{#407217}