Markers are barely visible with some themes

[danpla]

Makers are barely visible with some themes, or even completely invisible in case of Solarized (light) and Retro. The full list is:

• Bespin
• Dark Colors
• Delt Dark
• GitHub
• Kugel
• Monokai
• Railcasts
• Ralesk's MC
• Retro
• Solarized (light)
• Steampunk
• Tango
• Tinge

[codebrainz]

This is probably related to geany/geany#916 and geany/geany#917. In the meantime, if you would like to put together a pull request to select better colours, it would be welcome.