Changes to Custom.less

I made some changes to Custom.less to:
* improve the legibility of type in the Network panel (including the popover styles
* "DroidSansMono" wasn't appearing in the Console for me, so I activated it for all ".monospace" and "source-code" (including the Elements panel content).
* style the line numbers when present, e.g., Source views
* Style the file list view left-side panel on the Resources and Source panels
* Style the Elements panel sidebar content, including Canary's "Split panels vertically when docked to right" preference