I did notice when working in Cubasish or Clear Gray color themes,
in the Edit Window Rulers area the " < + > " symbols after the range, location and arrangement markers ( to add or jump ), are not visible.
Similar with Tempo & Time Signature missing the add " + " symbol.
Ardour version 9 on Debian 13 trixie / KDE.
Not a showstopper, but maybe someone knows how to fix this.