add "fill" action to the CLI, consolidate config generation

wiki/THEMES.md

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+*Note*: this document was written prior to `v1.0.0` and may not longer be an
+accurate depiction of the `monobiome` internals.
+
+# Themes
+The currently available theme variants across biomes (`default`, `grassland`,
+`tundra`, `savanna`), harshness (`hard`, `default`, `soft`), and scheme
+(`light`, `dark`) are shown below, displaying a few lines from a Python file in
+Neovim.
+
+You can find the 24 raw screenshots for each theme in `images/theme_caps` under
+the repository root (with filenames `<harshness>-<biome>-<scheme>.png`).
+
+## Default (gray) variants
+![Default light and dark variants](images/biome_variants/default_variants.png)
+
+## Grassland biome
+![Grassland light and dark variants](images/biome_variants/grassland_variants.png)
+
+## Tundra biome
+![Tundra light and dark variants](images/biome_variants/tundra_variants.png)
+
+## Savanna biome
+![Savanna light and dark variants](images/biome_variants/savanna_variants.png)