Add dark night theme (UnchartedBull#1424)

* Add dark night theme

* Apply style also to confirm modal

themes/dark-nights/README.md

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+# Dark Nights
+
+A dark theme with customizeable accent color which matches my octoprint theme.
+
+## Customize
+
+To customize the accent color edit the hex-codes at the beginning of the `custom-styles.css`.
+
+```css
+:root {
+    /* If a other accent color is more to your personal liking, change it here. ;) */
+    --accent: #d32f2f;
+    --accent-transparent: #d32f2f24;
+    ...
+}
+```
+
+Make sure to change also the transparent accent color.
+
+## Screenshots
+
+### Dashboard
+
+![Dashboard](screenshots/dashboard.png)
+
+![Dashboard](screenshots/temp-adjust.png)
+
+### Files
+
+![Dashboard](screenshots/file-browser.png)
+
+![Dashboard](screenshots/file-selection.png)
+
+![Dashboard](screenshots/file-preview.png)
+
+### Filament change
+
+![Dashboard](screenshots/filament-temp.png)
+
+![Dashboard](screenshots/filament-purge.png)
+
+### Printing
+
+![Dashboard](screenshots/printing-percent.png)
+
+![Dashboard](screenshots/printing-preview.png)
+
+![Dashboard](screenshots/printing-adjust.png)