Steps to do it:
Extract all files from "SeventySix - Interface.ba2" and "SeventySix - Interface_en.ba2"
Open "hudmenu.swf" with JPEXS Free Flash Decompiler. If it asks to open other SWF files, click "yes to all".
Find image 1816 in images folder (images\DefineBitsLossless2 (1816))
Click the Replace button
Select "Transparent pixel.png" file from this comment
Save the modified SWF file
Create a ba2 archive with Creation Kit or Cathedral Assets Optimizer
Copy to Data folder
Add the ba2 file to sResourceArchive2List in Fallout76Custom.ini
Transparent pixel.7z