Every option in nbpreview has an associated environmental variable that can be set to provide a default value. For example, to set the theme to 'material', run:

% nbpreview --theme='material' notebook.ipynb

To apply the 'material' theme without having to specify it in the --theme option, set the environmental variable associated with the command-line option. The environmental variables for each option are explicitly listed at the end of the command-line usage. They may also be found in the --help message under env var:.

% nbpreview --help

  -t, --theme
                                  The theme to use for syntax highlighting.
                                  Call '--list-themes' to preview all
                                  available themes.  [env var:
                                  NBPREVIEW_THEME; default: dark]

In the case of --theme, the environmental variable is NBPREVIEW_THEME. Set it by running

% export NBPREVIEW_THEME='material'

Now, whenever nbpreview is run, it will automatically set the --theme value to 'material'. To set this permanently, set the environmental variable in the shell’s startup file—such as ~/.zshrc, ~/.zshenv, ~/.bashrc, ~/.bash_profile, etc. Environmental variables set the new default for nbpreview, but they can still be overridden anytime by manually the relevant command-line option.