A way to make lines 'finite' objects? Like turn them into a rectangle for example

Hi, I always feel like such a dill using SketchUp, I use it so infrequently I never make any progress. So apologies for the dumb question. In the uploaded image I created a nested group of Fibonacci sequences as a layout for a room divider I am designing. I’m trying to find an easy way to select all those internal lines and expand them out to say 15mm widths centralised about the current lines. Probably easiest to look at the version without the spirals to make it simpler. Thanks!

The basic manual way would be to use the Offset tool to offset by7.5mm in one rectangle then double click the rest to repeat, select the border and offset out 7.5mm and remove the centre lines.

Thanks Box for the fast reply! I can select a rectangle and offset by 7.5mm OK, but I don’t know what it means to double click the rest and repeat, I am sorry, I’m so not good at this! I got a Mac too if it changes the way I would make a selection.

With nothing selected and using the offset tool, just double click on each face.

wow! That is something really fundamental about the tools that I have forgotten! Thank you thank you thank you!