I believe you’re referring to targeting a named style with Custom CSS which is covered in the CSS Wiki you linked to earlier:
I would note that we do not offer official staff support for Custom CSS. Of course, we keep an eye on the community and respond where appropriate, but otherwise we defer to the community.
That being said, when seeking support with Custom CSS, I would strongly recommend sharing as much detail as possible. For example, sharing the whole set of custom CSS rather in addition to the specific snippet you’re interested in can be immensely helpful. Since you can customize almost anything with the Custom CSS, it’s possible something else in your Custom CSS is impacting your snippet. In other words, these things generally cannot be reviewed in a narrow scope, but often need the context of what the rest of the CSS is doing or any customizations you’ve made to your dashboard/theme/etc.
If you’re not familiar with Custom CSS, reading through the first couple of pages on a CSS Tutorial could be helpful as well. 
Some of the fundamental principles like the fact that CSS is cascading, there’s a concept of precedence / weight based on the specificity of the selectors, and that custom CSS applied after the base system CSS can make some of the concepts make much more sense
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That being said, for sizing icons, I tend to prefer a much simpler syntax:
.tile .icon svg { font-size: 6em; }
The standard disclaimer applies…
Please note that any custom CSS snippets provided are not officially supported and may need to be adjusted with potential future changes to the SharpTools web app.