Improve visibility of \p in doxygen html, solves #1580

Codex assisted.

documentation/CMakeLists.txt

@@ -105,6 +105,7 @@ if(FLTK_BUILD_HTML_DOCS)
   set(GENERATE_LATEX NO)
   set(EXTRA_SECTIONS "HTML_SECTIONS")
   set(LATEX_HEADER   "")
+  set(HTML_EXTRA_STYLESHEET "${CMAKE_CURRENT_SOURCE_DIR}/fltk.css")
   set(DOXYFILE       "Doxyfile")
   set(LOGFILE        "${CMAKE_CURRENT_BINARY_DIR}/${DOXYFILE}_error.log")
 
@@ -155,6 +156,7 @@ if(FLTK_BUILD_PDF_DOCS)
   set(GENERATE_LATEX YES)
   set(EXTRA_SECTIONS "LATEX_SECTIONS")
   set(LATEX_HEADER   "${CMAKE_CURRENT_BINARY_DIR}/fltk-book.tex")
+  set(HTML_EXTRA_STYLESHEET "")
   set(DOXYFILE       "Doxybook")
   set(LOGFILE        "${CMAKE_CURRENT_BINARY_DIR}/${DOXYFILE}_error.log")
 

documentation/Doxyfile.in

@@ -1172,7 +1172,7 @@ HTML_STYLESHEET        =
 # list). For an example see the documentation.
 # This tag requires that the tag GENERATE_HTML is set to YES.
 
-HTML_EXTRA_STYLESHEET  =
+HTML_EXTRA_STYLESHEET  = @HTML_EXTRA_STYLESHEET@
 
 # 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