Add dark theme

frontend/src/components/settings/Themes.vue

@@ -0,0 +1,18 @@
+<template>
+  <select v-on:change="change" :value="theme">
+    <option value="">{{ $t('settings.themes.light') }}</option>
+    <option value="dark">{{ $t('settings.themes.dark') }}</option>
+  </select>
+</template>
+
+<script>
+export default {
+  name: 'themes',
+  props: [ 'theme' ],
+  methods: {
+    change (event) {
+      this.$emit('update:theme', event.target.value)
+    }
+  }
+}
+</script>