Using userChome.css to change to color of a sent(outing) message in Thunderbird

I’m running Thunderbird 147.0.1 (64-bit) on both my desktop and laptop, both using Windows 11 Version 25H2 (Build 26200.7705).

I’m trying to highlight sent (outgoing) messages where the correspondent and recipient are different (i.e., not sending to myself). When this condition is met, I want the subject line to appear green.

My logic is:

• Match outgoing mail where the correspondent column contains one of my addresses
• Exclude rows where the recipient column also contains one of my addresses
• Apply formatting only to the subject column

tr:has(

td.correspondentcol-column[title*="@pm.me"],

td.correspondentcol-column[title*="@protonmail.com"],

td.correspondentcol-column[title*="@passinbox.com"]

):not(

:has(td.recipientcol-column[title*="@pm.me"]),

:has(td.recipientcol-column[title*="@protonmail.com"]),

:has(td.recipientcol-column[title*="@passinbox.com"])

)

td.subjectcol-column {

color: var(--tb-green) !important;

font-weight: 600 !important;

background-color: #e8f5e9 !important;

}

Same Thunderbird version, same Windows build, same CSS file, same mail account structure.

Is there something on the desktop system that could prevent this from working?

For example:

• about:config setting related to CSS or :has() support
• Layout or column differences affecting selectors
• Rendering engine differences between installations
• UI density / table layout mode differences

My expectation is that this logic should behave identically on both systems, so I suspect a configuration or UI state difference.

Any guidance would be appreciated.