Did you know that you can change the style of existing layers (layers that you did not create yourself and for which you have no rights to edit)?
In order to do that, you have to add layers to the personal theme (see here how to create a new theme), where you can set style for each layer individually. Custom style is applied only within your personal theme.
Once you have created new theme and added some layers you may apply custom style to individual layers within the Theme.
Click on Edit layer`s style icon . The style editor wizard opens, where you can set custom style (styling option depends on the type of layer`s geometry - for more, see Layer styles).
In our case the type of geometry is "line". You can set different style parameters, such as: line style, line color, line width, opacity, ...
For advanced style options open Advanced editor, where you can manipulate the layer`s style by applying specific script rules.