Starting in V15, a custom symbol can be filled, which is useful when used with symbol colors, see Element Colors for details.
For example, see this schematic using filled symbols and colors to help visually identify elements for various parts of the design (input vs. output, DC vs. matching, etc).
When creating or editing a symbol in the symbol editor, you will need to draw the shape as a Rectangle.
After drawing the rectangle, it will draw without a fill.
Right-click on the shape and select Toggle Fill to make the shape filled.
Finally, you will have to change the element you want to use the newly created symbol. Double click on the element in the schematic and find the Symbol tab to assign symbols to elements.