New Here! Sorry if this has been asked already…
How do I change the color theme of a map?
How to export the map as a high resolution image for printing?
There are no colour themes in the map; they are imposed by rendering software.
To change the colour scheme in a rendered map, you will need to install the tool chain used to render the map (there is more than one possible tool chain) and, typically, change the style sheet it uses. (It is possible that people run services allowing you to run a tool chain “in the cloud” with a custom style sheet, but I’m not specifically aware of them. There are certainly people who will generated custom renderings, at a price. I believe the people who provide the cycling and transport renderings offer this service.
If you want high resolution in terms of very small aliasing artefacts, even the main website allows you to export PDF. If you really mean you want a map of a very large area, with fonts that are small, compared with the total area, you can simply splice together bitmap tiles.
For more information, see http://wiki.openstreetmap.org/wiki/Rendering
Can you give an example of what it is you want to try and change, and where?
If you need the different colour scheme for printing, you could look at http://maperitive.net/ or https://maposmatic.osm-baustelle.de/