You can use the tools from the Style tools to change the graphical attributes (color, thickness and line-pattern) of objects.
The Style tools contains these tools:
To change the color, line thickness, or line style of an object, pick up the tool by clicking on its cell and then click on the object you want to apply the tool to.
If the tool is a cycle tool, such as Cycle arc head size, click several times until the object has the property you want.
If several objects are placed on Magnetic guidelines, you can apply the style tool to the guideline to apply it to all objects on the guideline.
If you want more information about using these tools, then visit the Style tools page.