Negative margin property breaks things, depending on applied-to elements

 * Cinnamon version - 4.4.8
 * Distribution - Linux Mint 19.3
 * Graphics hardware *and* driver used - "Intel Corporation Device 3e9b" (HD Graphics) + GeForce GTX 1050 Mobile, set to the former via NVIDIA X Server Settings; as a result the driver manager doesn't show anything, all I know is it's the default open-source one
 * 64-bit

Issue
Using a negative margin in a theme (which should be allowed AFAICT) can cause a variety of glitches. The two I found so far were:

• when applied to .applet-box horizontally on a horizontal panel (didn't check vertical), makes everything after the negative margin disappear
• when applied to .calendar-day-heading (tested with margin-top), changing to the negative-margin-containing theme doesn't cause an update to the panels

Steps to reproduce

1. Copy one of the default themes to ~/home/.themes and change its name.
2. In cinnamon.css, add a margin(-left, -right, etc.) property with a negative value to one of the classes
3. Switch to the modified theme.

Expected behaviour
https://www.quirksmode.org/blog/archives/2020/02/negative_margin.html

Other information
I wanted to use it to nudge the leftmost two applets on my panel closer together without moving anything else, there's your use case. I ended up using :last-child instead, since my panel's left side only has the two applets.

Start by reproducing the issue with a modified theme's cinnamon.css, testing negative margins on .applet-box and .calendar-day-heading. Trace how those selectors affect panel layout and theme switching; done means negative margins render without hiding later elements and switching themes updates the panels correctly.