Provide directions to change the theme (#585)

* Add directions to change themes

The page 'general' was the first place I looked when I
wanted to change the theme. Not managing to find it,
I went on a search throughout various issues ( #46 , for
example) on Github before locating it.

Now I hope to spare others the search

* Hint the user about themes

When I was on this page, it wasn't immediately apparent
to me that the SettingsListItem was clickable (I thought it
was a list rather than a button)

routes/_pages/settings/general.html

@@ -25,6 +25,10 @@
     </div>
   </form>
   
+  <h2>Themes
+  </h2>
+  <p>Changes to the theme (colours) of Pinafore can be made in <i>Settings &gt; Instances &gt; *your instance*</i>
+  </p>
 </SettingsLayout>
 <style>
   .ui-settings {

routes/_pages/settings/instances/index.html

@@ -2,7 +2,10 @@
   <h1>Instances</h1>
 
   {#if $isUserLoggedIn}
-  <p>Instances you've logged in to:</p>
+  <p>Instances you've logged in to:
+  <br />
+    (click to change themes/colours)
+  </p>
   <SettingsList label="Instances">
     {#each $loggedInInstancesAsList as instance}
       <SettingsListItem offsetForIcon={instance.name !== $currentInstance}