Fixes selects having unreadable text on some browsers/OSes.

Currently some Browser/OS configurations allow <option> to be somewhat styled, and it leads to them being unreadable on darker themes due to color being inherited from body to a light color but background still being white. See image below:

Currently known browsers/os that has this issue:

• Firefox/Windows (Linux version uses GTK dropdown and therefore cannot be styled)
• Chrome/Linux

While i was at it, i added trivial theme switcher theme preview.

Closes #123 (closed)

src/components/style_switcher/style_switcher.vue

@@ -3,7 +3,10 @@
     <div>{{$t('settings.presets')}}
       <label for="style-switcher" class='select'>
         <select id="style-switcher" v-model="selected" class="style-switcher">
-          <option v-for="style in availableStyles" :value="style">{{style[0]}}</option>
+          <option v-for="style in availableStyles" :value="style" :style="{
+                  backgroundColor: style[1],
+                  color: style[3]
+                  }">{{style[0]}}</option>
         </select>
         <i class="icon-down-open"/>
       </label>