[fractal] Add css for headlines

[Alejandro Domínguez <aledomu src gnome org>]

commit 1ec9ca3955a4a1eedb6a6719a071fd498c7dd0f5
Author: Maximiliano Sandoval R <msandova protonmail com>
Date:   Sat Oct 10 23:54:46 2020 +0200

    Add css for headlines

 fractal-gtk/res/app.css | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)
---
diff --git a/fractal-gtk/res/app.css b/fractal-gtk/res/app.css
index 3a6c598d..c5ef1af4 100644
--- a/fractal-gtk/res/app.css
+++ b/fractal-gtk/res/app.css
@@ -412,6 +412,22 @@ box.folded-history
   padding-right: 6px;
 }
 
+.h1, .h2, .h3, .h4, .h5, .h6 {
+  font-weight: bold;
+}
+
+.h1 {
+  font-size: xx-large;
+}
+
+.h2 {
+  font-size: x-large;
+}
+
+.h3 {
+  font-size: large;
+}
+
 #clip-container {
   border-radius: 6px;
 }