Design updates for usage list view

[jldec]

• better separation of user avatar
• distinguish more effectively between prebuilds and workspaces
• refine sidebar elements
• improve dark/light mode
• color improvements

See design specs.

[gtsiolis]

Moved this to Product Design / Scheduled. 🥯

[gtsiolis]

Closing this as all points mentioned above have been resolved except the one regarding the sidebar elements which is being tracked in #11685.

Future improvements on the sidebar elements include #13288, #13290, etc.

See also relevant discussion (internal).