How can I change the colour of the highlight line in M-x for the light ef-themes?

On a number of the light ef-themes the completion lists for C-x b and M-x use a dark highlighting line over font colors that have to be dark and makes the unreadable. How can I change it? Does it mean deriving a new theme from the selected theme or is there some setting to override it? Does the colour for the highlight line have a name? I am going through the list displayed by the ef-themes-list-colors command but can't find a likely symbol name.