Add theme variants

- Use CSS variables instead of inherence
- Rename default theme to "Light - Serif"
- Rename Black theme to "Dark - Serif"
- Rename "Sans-Serif" theme to "Light - Sans Serif"
- Add "System" theme that use system preferences: Dark or Light
- Add Serif and Sans-Serif variants for each color theme

model/user_test.go

@@ -39,7 +39,7 @@ func TestValidateUserModification(t *testing.T) {
 		t.Error(`There is no changes, so we should not have an error`)
 	}
 
-	user = &User{Theme: "default"}
+	user = &User{Theme: "system_serif"}
 	if err := user.ValidateUserModification(); err != nil {
 		t.Error(`A valid theme should not generate any errors`)
 	}