Migrate to bulma-css-variables and introduce dark mode (#954)

Co-authored-by: Adrian Simmons <adrian@perlucida.co.uk>
Co-authored-by: Dominik Pschenitschni <mail@celement.de>
Co-authored-by: kolaente <k@knt.li>
Reviewed-on: https://kolaente.dev/vikunja/frontend/pulls/954
Reviewed-by: dpschen <dpschen@noreply.kolaente.de>
Reviewed-by: konrad <k@knt.li>
Co-authored-by: adrinux <adrian@perlucida.co.uk>
Co-committed-by: adrinux <adrian@perlucida.co.uk>

src/i18n/lang/en.json

@@ -112,6 +112,15 @@
         "disabled": "Disabled",
         "todoist": "Todoist",
         "vikunja": "Vikunja"
+      },
+      "appearance": {
+        "title": "Color Scheme",
+        "setSuccess": "Saved change of color scheme to {colorScheme}",
+        "colorScheme": {
+          "light": "Light",
+          "system": "System",
+          "dark": "Dark"
+        }
       }
     },
     "deletion": {