fix isort lint issue

Author: Gopinath-Mahendiran

zulipterminal/themes/zt_dark.py

@@ -8,6 +8,7 @@
 For further details on themefiles look at the theme contribution guide.
 """
 from pygments.styles.solarized import SolarizedLightStyle
+
 from zulipterminal.themes.colors_gruvbox import DefaultBoldColor as Color