reports.core: Add BaseODS.bgcolor_style() method.

conservancy_beancount/reports/core.py

@@ -896,6 +896,18 @@ class BaseODS(BaseSpreadsheet[RT, ST], metaclass=abc.ABCMeta):
 
     ### Styles
 
+    def bgcolor_style(self, color: str) -> odf.style.Style:
+        key =f'BGColor{color.lstrip("#")}'
+        try:
+            retval = self._style_cache[key]
+        except KeyError:
+            props = odf.style.TableCellProperties(backgroundcolor=color)
+            retval = odf.style.Style(name=key, family='table-cell')
+            retval.addElement(props)
+            self.document.styles.addElement(retval)
+            self._style_cache[key] = retval
+        return retval
+
     def border_style(self,
                      edges: int,
                      width: str='1px',