WIP: theme: SASS restructuring, theme fixes and design updates

This merge request aims to complete a few goals:

1. Improve the structure and organization, by:

• rewriting the SASS to be modular; makes editing and finding style definitions for specific parts of the shell easier
• make use of more SASS variables so there's fewer values to track
• nest classes as much as possible as to mimic it's UI structure

2. Include fixes or add support for the dual color variations (i.e. light and dark)

• where needed, insert all the needed $variant == 'light' styles

3. Iron out some long-standing but minor visual bugs

• popovers > menus: refinements to menuitems' padding and borders
• sliders: fixes issues with the borders and legibility of the handle

4. Tackle some of the design goals for the shell that can be achieved in CSS

Calendar Redesign

• redesigned Calendar popover
• updated and richer notifications
• general consistency of elements

Search Redesign

• background legibility of search results, fixes #288 (closed)