You have to remap the system colors. View/Display Settings/System Colors. On the Graphics tab, change the color assigned to letter & highlight edge. Play around there, it will only affect the current session. Once you have what you want, add the config option system_colors_file full_path. Search the fundamentals help section for more on system colors.
you could select all the dim. right click onit andgo to properties dim prop./text style orgo to format/text-style gallery/modifytext style/select text-dim and change the color, but this will be only a temporary solution to your problem. Maybe somebody else know were to find it in your config.pro.