The Quake Retexturing Project is a set of hi-quality textures replacements to use with the Quake engine modifications. It's not the biggest set available, but seems like it's the only one supported by Tenebrae.
Download 'qrp-maptextures-2007-10-06r2-7files.zip' and unpack it to separate folder.
Open that folder, find a file 'qrp-maptextures-2007-10-06r2.pk3' and rename it to 'qrp-maptextures-2007-10-06r2.zip'.
Unpack 'qrp-maptextures-2007-10-06r2.zip' to folder 'tenebrae' in original Quake folder (this was 'E:\games\quake\tenebrae' in my case).
As a result, the folder 'textures' should appear in folder 'tenebrae'. Rename folder 'textures' to 'override'. The resulting files structure should look like the one on the screenshot.