Divider between treeview and text

In doxygen the setting `HTML_COLORSTYLE` has been introduced with as default `AUTO_LIGHT` but due to the current strategy of CGAL in respect to HTML stylesheets the divider between the treeview and the text is not properly  visible anymore. Due to the same strategy the setting to `AUTO_LIGHT` does not make sense for CGAL.
When setting `HTML_COLORSTYLE=LIGHT` the divider is properly visible again.

Documentation/doc/resources/1.9.3/BaseDoxyfile.in

@@ -1308,6 +1308,23 @@ HTML_STYLESHEET        = ${CGAL_DOC_RESOURCE_DIR}/stylesheet.css
 
 HTML_EXTRA_STYLESHEET  =
 
+# The HTML_COLORSTYLE tag can be used to specify if the generated HTML output
+# should be rendered with a dark or light theme. Default setting AUTO_LIGHT
+# enables light output unless the user preference is dark output. Other options
+# are DARK to always use dark mode, LIGHT to always use light mode, AUTO_DARK to
+# default to dark mode unless the user prefers light mode, and TOGGLE to let the
+# user toggle between dark and light mode via a button.
+# Possible values are: LIGHT Always generate light output., DARK Always generate
+# dark output., AUTO_LIGHT Automatically set the mode according to the user
+# preference, use light mode if no preference is set (the default)., AUTO_DARK
+# Automatically set the mode according to the user preference, use dark mode if
+# no preference is set. and TOGGLE Allow to user to switch between light and
+# dark mode via a button..
+# The default value is: AUTO_LIGHT.
+# This tag requires that the tag GENERATE_HTML is set to YES.
+
+HTML_COLORSTYLE        = LIGHT
+
 # The HTML_EXTRA_FILES tag can be used to specify one or more extra images or
 # other source files which should be copied to the HTML output directory. Note
 # that these files will be copied to the base HTML output directory. Use the