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.
Since recenctly (I don’t recall when exactly) there are issues with multiple themes regarding how text colors are inverted depending on the background color, either these themes must be adjusted or maybe some contrast threshold can be tweaked in the toolkit.