There is no such thing as "line thickness" in Blender; Blender is a 3D modelling software, not a 2D Illustration or Graphic design application.
Only way I see you can simulate a stroke in Blender is using beveling either through a simple Bevel Depth or through a Bevel Object
You batch set Bevel Objects for more than one curve at a time by pressing Alt while clicking the field or the picker icon, or you can copy the from an object to others by right clicking and choosing the option Copy to Selected.
If possible join all curves into one single object and then assign the Bevel Object in a single step, otherwise manually assign one by one.
You can just draw a simple straight line segment curve object and assign it as bevel object to simulate a "flat thickness". You can control final thickness per vertex by changing the per-vertex radius property.
Otherwise, if you don't mind a 3D-like circular-section bevel, you could just select all objects with the same thickness and bevel all curves simply with the desired Bevel Depth. This property can be batch assigned to all selected objects by pressing Alt while clicking the value button.