Feature: color scheme for white terminals

Right now, `diskonaut` doesn't look very nice on white terminals. It would be nice to add some other color schemes. Ideally allowing the user to both provide a command line flag (something like `--colorscheme white`) and to change it while the app is running (to be able to better judge what works best for them).

No files or tests are named. Start by locating the command-line argument handling and the runtime terminal-color rendering, then determine how a selectable color scheme could be provided at startup and changed while the app runs. Done means white terminals are readable and both requested controls work.

Written by the indexing model from the issue text.