We have an example for overriding font in our docs. You are better off declaring this in a UI Template that is of type "CSS (UI Scoped). Then no need for the <style> tags.
You should then be able to reference "Segment" in a second CSS declaration. There is an example in the docs
.nrdb-ui-text sets the font-family to Helvetica. Can't see any good reason doin so but if user wants to change whole page, the ui_text is the thing to adjust separately.
inspect the dashboard yourself using the browsers devtools and you will find the class for the class for the value part is .nrdb-ui-text-value
To further limit the CSS from affecting ALL text nodes, you should add a class to ONLY the ui-text widget you want to have this font (e.g. seven-seg) then address it like this: