Changes from the original layout:

• Top blue header reduced from 60px to 32px (on all Waze pages, not just the editor).
• White-space (blue-space?) in top header reduced.
• Header logo and links moved to the left instead of center.
• Added a reminder to press Shift + F to hide the top header.
• Sidebar shrinked by about 100px.
• Horizontal whitespace in the sidebar reduced.
• Vertical whitespace in the sidebar reduced.
• Reduced the height of sidebar tabs.
• Adding a gray color to indicate driving sessions that are no longer available.
• Removed the upper limit of 2048px for the map editor size. Not needed anymore, latest Waze editor has already removed this limit.
• Reduced the lower limit of 620px to 600px for the map editor size.
• Light blue toolbar reduced from 60px to 38px.
• Point/area buttons from the toolbar drop-down menus reduced from 40px to 28px.
• Moved the header of house-number-editing UI to the top, increasing the editable space.

Inspired by http://freestyler.ws/style/76897/waze-minimize-useless, but written from scratch.

Also posted to the https://www.waze.com/forum/viewtopic.php?f=819&t=77913, to https://www.reddit.com/r/waze/comments/2grnhx/waze_editor_layout_tweaking_custom_css/, and added to the https://wiki.waze.com/wiki/Community_Plugins,_Extensions_and_Tools#WME_layout_tweaking_user_style