This is where this tool is designed for: select a theme file and click "Edit". You will get a page with all elements and the assigned colors; by clicking a color you can change it. When you are finished you can download the modified theme file.
The tool uses the element names from the file. I don't know exactly which element defines what; it might need some experimenting. An important one for zūmo users is "Map_RTE_ACTV_LEG_CLR", there you can set the width of the active route.
Some elements have the options "Scale" and "Border". "Scale" is (presumably) the ratio of the width of a line element compared to the default value. Enter "2" and the line will be twice as wide. "Border" sets the border with of line elements.
Some elements without a "Border" option offer the possibility to set a different border color, and some line elements with a "Border" option don't offer the option to set the border color. Don't know why.... I don't have a zūmo 350, so I can't check.
The theme file usually contains several profiles: "Guidance", "Browse", "Traffic", "Turn Preview", "Route Preview" and "Pedestrian". By unchecking the option "Same style for every profile (default)" you will get to see the separate profiles, enabling you to select different colors for every profile. Default this option is checked; in that case the color choices for the first profile are copied to the other profiles. In the original theme files I've seen the colors are the same for every profile.
Using the button "Save to file" you can download the modified theme file; put it on your zūmo and check the result.