A solution for that is to use a dark wallpaper with a strong Ambience colour: AllBout's Tourmaline high-contrast Ambiences do exactly that and come in 6 different colours.
Those Ambiences can be downloaded all together in a ZIP-file or individually.
Package (all Ambiances):
Tourmaline high-contrast Ambience Package
Single download:
Yellow: Download here
Aloe Blue: Download here
Poppy Red: Download here
Green: Download here
Purple: Download here
Pink: Download here
White: Download here
AllBoutJolla: Download here
[Update 19.06.14] Revised wallpapers (removed ugly paint-job)
Tip by epdm2be for White Ambience/Highlights (open terminal and enter):
dconf write /desktop/jolla/theme/color/highlight "'#ffffff'"
dconf write /desktop/jolla/theme/color/secondaryHighlight "'#aaaaaa'"
You might also find some more high-contrast Ambiences here at together.jolla.com. Enjoy the sunny days!