Add SETI theme

app/models/theme.rb

@@ -1,6 +1,7 @@
 class Theme < Struct.new(:name, :label)
 
   AVAILABLE = {
+    'seti' => 'Seti',
     'tango' => 'Tango',
     'solarized-dark' => 'Solarized Dark',
     'solarized-light' => 'Solarized Light',

app/views/docs/embedding.html.md

@@ -87,6 +87,7 @@ The `theme` option allows overriding a theme used for the terminal.
 It defaults to a theme set by the asciicast author (or to "tango" if not set
 by the author).  There are 3 available themes:
 
+* seti
 * tango
 * solarized-dark
 * solarized-light

spec/services/asciicast_image_generator_spec.rb

@@ -6,7 +6,7 @@ describe AsciicastImageGenerator, needs_phantomjs_2_bin: true do
   let(:template_renderer) { ApplicationController.new }
 
   describe '#generate' do
-    let(:asciicast) { create(:asciicast) }
+    let(:asciicast) { create(:asciicast, theme_name: 'tango') }
 
     def rgb(color)
       [ChunkyPNG::Color.r(color), ChunkyPNG::Color.g(color), ChunkyPNG::Color.b(color)]