fix crash when converting chronicles example using bare theme

lib/asciidoctor/pdf/converter.rb

@@ -482,6 +482,7 @@ def prepare_theme theme
         theme.description_list_term_spacing ||= 0
         theme.description_list_description_indent ||= 0
         theme.table_border_width ||= 0.5
+        theme.thematic_break_border_color ||= '000000'
         theme.image_border_width ||= 0
         theme.code_linenum_font_color ||= '999999'
         theme.callout_list_margin_top_after_code ||= 0
@@ -2414,7 +2415,7 @@ def @bounding_box.move_past_bottom *args # rubocop:disable Lint/NestedMethodDefi
               align: :left,
               inline_format: false,
               margin_bottom: @theme.description_list_term_spacing,
-              style: @theme.description_list_term_font_style.to_sym
+              style: (@theme.description_list_term_font_style || 'bold').to_sym
             category.terms.each {|term| convert_index_list_item term }
             # NOTE: see previous note for why we can't use margin_bottom method
             if @theme.prose_margin_bottom > y - reference_bounds.absolute_bottom

spec/converter_spec.rb

@@ -341,6 +341,13 @@
         (expect (pdf.find_text 'one')[0]).not_to be_nil
       end
 
+      it 'should not crash if theme does not specify any keys when converting chronicles example' do
+        input_path = Pathname.new example_file 'chronicles-example.adoc'
+        pdf = to_pdf input_path, attribute_overrides: { 'imagesdir' => '@', 'pdf-theme' => (fixture_file 'bare-theme.yml') }
+        (expect pdf.pages).to have_size 14
+        (expect (pdf.page 1).text).to include 'Documentation Chronicles'
+      end
+
       it 'should convert background position to options' do
         converter = Asciidoctor::Converter.create 'pdf'
         {