You're hitting a bug in the oxygen-gtk theme. It's easy to work around this problem: switch "System Settings>Application Appearance>GTK+ Appearance>Widget Style" to anything other than "oxygen-gtk" - "Adwaita" or "HighContrast", for example. When the "Widget Style" has been switched away from "oxygen-gtk", run the .bin installer file for Crossover in terminal, again.
After this succeeds, you can switch the "Widget Style" back to "oxygen-gtk" - this bug only affects the initial installation screen, the rest of Crossover's functions will work, post-install.
You can read read more about this bug here. It has been reported as fixed. If you continue to experience this issue or are unable to update, you can set this environment variable prior to launching the CrossOver installer: