Update theme to use only free fonts by default

Author: tingerrr

src/theme.typ

@@ -74,11 +74,11 @@
 #let default = (
   fonts: (
     serif: ("Linux Libertine", "Liberation Serif"),
-    sans:  ("Liberation Sans", "Helvetica Neue", "Helvetica"),
+    sans:  ("Liberation Sans", "TeX Gyre Heros"),
     mono:  ("Liberation Mono",),
 
     text:     ("Linux Libertine", "Liberation Serif"),
-    headings: ("Liberation Sans", "Helvetica Neue", "Helvetica"),
+    headings: ("Liberation Sans", "TeX Gyre Heros"),
     code:     ("Liberation Mono",),
   ),
   colors: (