Improve focus handling in notebooks

[kieferrm]

• focus in large cells It's not clear which notebook cell is selected. Which will execute next on shift or ctrl-enter? #112096
• investigate play button changes when the cell has focus (such as background color)
• polish focus colors in out-of-the-box themes

[vscode-triage-bot]

Pinging notebook topic followers: @claudiaregio @rchiodo @greazer @DonJayamanne @jilljac

[greazer]

See comment I left in #112096 for a suggestion on default coloring for focused cells.

[miguelsolorio]

Closing issue as most are done, created #117302 to track the default themes colors for notebooks.