I'm not sure to understand your problem completely. Please explain more detailed. Do you want to set line properties for drawing new lines, or do you want to change existing lines? What means "it's the same for all layers"? Do the lines seem to look the same although you have set them differently?
Before drawing a line you set the properties in the line attributes tab. You can choose a certain type, or say "By Layer", then the value set in the layer attributes is valid.
"How to recap an ad on my mailbox when someone answered." Sorry I don't get what you mean. Maybe because English is not my native tongue.
You should study the manuals. Now you get there via Help - Online. At least it works now on LC 2.1.3. Or go to the WIKI.
Excuse me for being late
Firstly I am French and if this language is more family friendly, it will facilitate our exchanges because I too have trouble with English.
When, on the right column I choose to edit a layer, I do not find these settings automatically on the three buttons at the top left of the drawing area, even if this layer is the only visible in the right column.
I am German, don't speak any French. I have not had it in school at all, what I regret nowadays.
There are two ways to define line properties, by layer or directly. In the early days of CAD they had true ink pen plotters to print out a drawing. If you wanted to change line thickness or colour you had to change the pen first. So you had to organize the drawing in layers for different pens, plotting them one after the other. Wouldn't have been really necessary for the line type, but as this system had been established, it was used for that too.
With todays printing technology there is no need any more to organize your drawing this way, but it has become an established method. So you still can use layers not only to sort the drawing for certain contents, but also for line properties. But you don't have to.
If you draw a line with the pen properties bar (top left) set to red, it will be red even if the layer attributes are set to blue. If in the pen properties bar you set "By Layer" (Par Calque) , it will be blue, according to the layer attributes. That's why the layer default pen settings do not automatically show up in the top left pen properties bar. Two different things.
You are using an older version of LibreCAD. Try to install the latest version. It will look a bit different, the user interface has changed a bit. For instance, the default layer colour is shown in the layer list.