perl -i -p -e 's/Pridetree/Bridetree/;' /Applications/Sourcetree.app/Contents/Resources/Assets.car
You are correct in that ThemeEngine does not work on Catalina 10.15.
Thank you @Sterling Bourne ! It worked, but I had to use sudo.
Dear Atlassian please keep the pride support to yourselves, don't force us to watch it everyday! Thanks.