Modernize some panel styles and colors

[federicobond]

Description

This is a first pass at updating and modernizing panel styles. Takes some cues from the design language of the built-in Django Admin UI, especially table headers and colors.

I've also updated some column headers and placed units like ms beside each value instead of up in the headers. This improves legibility and allows expressing each value with appropriate units if necessary later.

Where appropriate, I've reduced the precision of some values displayed to a more meaningful value.

I would like to follow up this with specific improvements to the information layout of some panels.

Screenshots

Note: the Time column width has been fixed after the screenshot was taken.

Checklist:

• I have added the relevant tests for this change.
• I have added an item to the Pending section of docs/changes.rst.

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
debug_toolbar/panels
timer.py
debug_toolbar/panels/sql
utils.py
Project Total

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