fix volume bar border colors when using make_marketcolors

When make_marketcolors() is called with custom volume colors, it
updates marketcolors['volume'] but leaves marketcolors['vcedge']
pointing to the old base style value. This causes the equality check
in plotting.py (mc['volume'] == mc['vcedge']) to fail, so the
auto-darkening logic for volume bar edges is skipped and the old
base style colors are used instead.

Fix by syncing vcedge with volume whenever volume colors are changed
in make_marketcolors().

Fixes #676

Author: Carson Jones

src/mplfinance/_styles.py

@@ -346,6 +346,12 @@ def _check_and_set_mktcolor(candle,**kwarg):
             c   = _check_and_set_mktcolor(candle,**kwa)
             marketcolors.update([(kw,c)])
 
+    # Keep vcedge in sync with volume so that the auto-darkening
+    # logic in plotting.py still triggers after custom volume colors
+    # are applied (fixes issue #676).
+    if config['volume'] is not None or config['inherit'] == True:
+        marketcolors['vcedge'] = marketcolors['volume']
+
     if config['hollow'] is not None:
         marketcolors.update({'hollow':config['hollow']})