This particular text can be edited in the admin panel : Customize > Texts
Type this in the search bar : js.tagging.selector_all_tags
And validate the text
But to answer your question in a general context, editing a text is doable but tricky with css. I generally use the content selector in a ::before or ::after class.
https://www.w3schools.com/cssref/sel_before.asp
If we didn’t have the possibility to edit the text,I’m not an expert but I would do something like this.
First, I hide the all tags text:
summary.tag-drop-header .name {
font-size: 0;
}
Then, I create a new css class, with content to add the text I want, and I need to specify the font-size to override the previous setting:
summary.tag-drop-header .name::after {
content: "LFX Tools";
font-size: 15px;
}