An alternative workflow to the one presented by @cristian-ciupitu would be this:
- Create (or select) an object or a path with no fill. If you want, you can already set the stroke width (Shift+Ctrl+F), or fix this later.
- Duplicate (Ctrl+D) the object/path. This will create a duplicate on top of the original.
- Send the duplicate Lower (
Lower, or Page Down), so that you can...
- ... choose Select (
arrow on the toolbar, or F1) and Shift+Click the object/path. As the "Lowered" duplicate was already selected, this will select the two identical objects/paths on top of each other. The status line at the bottom of the window should confirm this.
- Now choose
Voila! Inner border!
You can now adjust the inner border width (Shift+Ctrl+F) if you want. Set the stroke width to twice your desired inner border width, as half of the stroke width will be clipped invisible.
If you think steps 3 and 4 are confusing, just drag around the two objects/paths to select them both, clip them as in step 5, and adjust the stroke width as desired. Works as well, but hey, clicking is cooler ;)