not inherited standard stylesheet

I use the developer toolbar on firefox for macosX.
I noticed, some weeks ago, I was missing the arrow in the navigation bar.
To let you understand without misunderstandings, I am talking about this arrow:
<button title="Previous activity" type="submit">
<span class="arrow">?</span>
<span class="accesshide"> Previous activity</span>
</button>

Well, by adding the right style to my theme
.arrow, .arrow_button input {
font-family: Arial,Helvetica,Courier,sans-serif;
}
all went right... but, why am I not inheriting this style from standard theme?
So I went to my developer toolbar and I selected CSS->View CSS.
What I got was a long page (http://myserver/moodle18/theme/standard/styles.php)
starting with:
http://myserver/moodle18/theme/standard/styles.php
/**************************************

• THEME NAME: standard
  *
• Files included in this sheet:
  *
• standard/styles_layout.css
• /mod/assignment/styles.php
• /mod/choice/styles.php
• /mod/data/styles.php
• /mod/lesson/styles.php
• /mod/pagemenu/styles.php
• /mod/quiz/styles.php
• /mod/scorm/styles.php
• /mod/workshop/styles.php
• /blocks/blog_tags/styles.php
• /blocks/calendar_month/styles.php
• /blocks/calendar_upcoming/styles.php
• /blocks/compliance/styles.php
• /blocks/course_menu/styles.php
• /blocks/course_summary/styles.php
• /blocks/login/styles.php
• /blocks/news_items/styles.php
• /blocks/page_module/styles.php
• /blocks/quiz_results/styles.php
• /blocks/rss_client/styles.php
• /blocks/search_forums/styles.php
• /course/format/page/styles.php
  **************************************/

Question and, maybe, bug: Why are not seen the other two standard stylesheet:
styles_fonts.css and styles_color.css?