Accordion Item Title in BOLD only when Accordion is open

You can use this code to Website Tools > Custom CSS.

li[data-is-open="true"] span.accordion-item__title {
    font-weight: bold;
}

Result

If you want to apply this for specific accordion block. First you need to find Accordion Block ID.
In my example, it is: #block-yui_3_17_2_1_1729387285192_9027

Next, use this CSS code

#block-yui_3_17_2_1_1729387285192_9027 li[data-is-open="true"] span.accordion-item__title {
    font-weight: bold;
}

If you want to apply this for specific accordion – but specific item. We will need to use CSS code like this

#block-yui_3_17_2_1_1729387285192_9027 li[data-is-open="true"]:nth-child(2) span.accordion-item__title {
    font-weight: bold;
}

this will bold accordion item 2 title only

In case you want to make underline, instead of bold, just replace

font-weight: bold;
with
text-decoration: underline;