Can you clarify what you mean by this? Did you mean that you are using a SharpTools Custom Tile with Custom HTML? In the case of a Custom Tile, the rendering of the tile happens in its own sandboxed context and the developer of the Custom Tile is fully responsible for the layout and sizing of various elements within the Custom Tile.
Since it’s rendered in a sandbox, Custom CSS from the Dashboard level can’t reach inside the sandbox of the Custom Tile. But the Custom Tile itself could be modified with HTML or CSS as desired.
You can target both system icons and custom image icons as @GrooveENERGY_ca showed here.
While they showed targeting each of these icon types separately, CSS allows you to use comma separated list of selectors. So with Groove Energy’s example, if you wanted to apply the same adjustments to both types, it could be combined like:
/* All on one line */
.tile .icon svg, .tile .icon img { /* your adjustments here */ }
/* Or separated so it's easier to read */
.tile .icon svg,
.tile .icon img {
/* your adjustments here */
}