doc: add a custom doxygen theme based on doxygen-awesome

Theme from doxygen-awesome-css with custom modifications based on the
pipewire.org website to use the same type of blue, grey, etc.

doxygen-awesome-css is MIT licensed, see
https://github.com/jothepro/doxygen-awesome-css

doc/Doxyfile.in

@@ -30,3 +30,4 @@ MACRO_EXPANSION        = YES
 EXPAND_ONLY_PREDEF     = YES
 PREDEFINED             = PA_C_DECL_BEGIN= \
                          PA_C_DECL_END=
+HTML_EXTRA_STYLESHEET  = @cssfiles@

doc/custom.css

@@ -0,0 +1,14 @@
+:root {
+	/* --page-background-color: #729fcf; */
+	--primary-color: #729fcf;
+	--primary-dark-color: #729fcf;
+	--header-background: #729fcf;
+	--header-foreground: rgba(255, 255, 255, 0.7);
+	--font-family: 'Source Sans Pro', 'Source Sans', sans-serif;
+	--code-background: #f5f5f5;
+	--code-foreground: #333333;
+	--fragment-background: #f5f5f5;
+	--fragment-foreground: #333333;
+	--fragment-keyword: #c7254e;
+	--fragment-link: #729fcf;
+}

doc/meson.build

@@ -41,7 +41,13 @@ foreach extra : extra_docs
   inputs += meson.source_root() / 'doc' / extra
 endforeach
 
+cssfiles = [
+  meson.source_root() / 'doc' / 'doxygen-awesome.css',
+  meson.source_root() / 'doc' / 'custom.css'
+]
+
 doxyfile_conf.set('inputs', ' '.join(inputs))
+doxyfile_conf.set('cssfiles', ' '.join(cssfiles))
 
 doxyfile = configure_file(input: 'Doxyfile.in',
                           output: 'Doxyfile',
@@ -53,7 +59,7 @@ if docdir == ''
 endif
 
 html_target = custom_target('pipewire-docs',
-                            input: [ doxyfile ] + inputs,
+                            input: [ doxyfile ] + inputs + cssfiles,
                             output: [ 'html' ],
                             command: [ doxygen, doxyfile ],
                             install: true,