Use the Body tag in front of it... be more specific:
body .thm-unit-test h3 {
font-size: 28px !important;
}
Or other parent elements...
Take some time to understand CSS Selector Priority:
Understanding CSS selector priority / specificity
A selector's specificity is calculated as follows:
count 1 if the declaration is from is a 'style' attribute rather than
a rule with a selector, 0 otherwise (= a) (In HTML, values of an
element's "style" attribute are style sheet rules.
These rules have no
selectors, so a=1, b=0, c=0, and d=0.) count the number of ID
attributes in the selector (= b) count the number of other attributes
and pseudo-classes in the selector (= c) count the number of element
names and pseudo-elements in the selector (= d) The specificity is
based only on the form of the selector.
the form "[id=p33]" is counted as an attribute selector (a=0, b=0,
c=1, d=0), even if the id attribute is defined as an "ID" in the
source document's DTD. Concatenating the four numbers a-b-c-d (in a
number system with a large base) gives the specificity.
In particular, a selector of the form "[id=p33]" is counted as an attribute selector
(a=0, b=0, c=1, d=0), even if the id attribute is defined as an "ID" in the source
document's DTD.
Concatenating the four numbers a-b-c-d (in a number system with a large base) gives the > specificity.
https://www.w3.org/TR/CSS2/cascade.html#cascading-order
I'd use an ID in front of it tbf.