Also allow default theme for documentation.
Also allow default theme for documentation.

Fix #134

untitaker/vdirsyncer,hobarrera/vdirsyncer,tribut/vdirsyncer,hobarrera/vdirsyncer,mathstuf/vdirsyncer,credativUK/vdirsyncer,tribut/vdirsyncer,untitaker/vdirsyncer,untitaker/vdirsyncer,mathstuf/vdirsyncer,credativUK/vdirsyncer