Mapping Center Answer:

The suggested procedure is to find a line symbol (like you have already) in an existing style, then copy that symbol to either a new style, or your personal style under the same folder type. In this case, it is a symbol that you are copying from the line symbol folder of one style to another line symbol folder.

After you have copied the symbol, then access it's properties by right-clicking the symbol and clicking Properties. The Symbol Editor window will appear and you will see all the properties for that symbol. If it is a complex symbol, it may have more than one layer to create a patterned line. If it is a simple symbol, it may still have more than one layer, but likely it will be comprised of solid lines.

Choose the layer of the symbol you want to modify and make your changes in color, width, offset, etc. Remember to save your symbol to the style so you can choose it when you want to use it in ArcMap.

Here is the online help topic for Modifying and saving symbols.