do not crash if the terminal does not have color support

tests/features/conftest.py

@@ -303,6 +303,7 @@ class DeferredRunner:
         self.color_pairs = {0: (7, 0)}
         self.screen = Screen(width, height)
         self._n_colors, self._can_change_color = {
+            'xterm-mono': (0, False),
             'screen': (8, False),
             'screen-256color': (256, False),
             'xterm-256color': (256, True),

tests/features/syntax_highlight_test.py

@@ -153,3 +153,8 @@ def test_syntax_highlighting_tabs_after_line_creation(run, tmpdir):
         h.press('Enter')
 
         h.await_text('foo\n    x\nx\ny\n')
+
+
+def test_does_not_crash_with_no_color_support(run):
+    with run(term='xterm-mono') as h, and_exit(h):
+        pass