Improve sidebar styles
Fix card background image variable
Only display Material border when menu collapsed.
Ensure padding doesn't break menus.
Adding column layout according to Material guidelines:
- Maximum of 3 columns in "web" layout, with 360px fixed width columns, 40px gutters and excess padding left and right.
- 2 columns in "tablet" layout, with variable size columns and 24px gutters.
- 1 columns in "phone" layout, with 16px gutters.