You could probably use a snippet similar to the one @smart_tomlinson shared.
So based on what you have so far, that would likely be:
.tile.--theme-style-supertilestyle .icon {
-webkit-filter: drop-shadow(0 0 3px #ff0)drop-shadow(0 0 10px #ff0)drop-shadow(0 0 20px #ff0);
filter: drop-shadow(0 0 3px yellow)drop-shadow(0 0 10px yellow)drop-shadow(0 0 20px yellow);
}
Please note that any custom CSS snippets provided are not officially supported and may need to be adjusted with future app updates.