The font is grey because it indicates that you have read this topic. It helps differentiate read/unread topics.
This is the default CSS:
.topic-list-item.visited a.title:not(.badge-notification), .latest-topic-list-item.visited a.title:not(.badge-notification), .category-topic-link.visited a.title:not(.badge-notification) {
color: var(--primary-medium);
}
You can replace --primary-medium by other color variables, such as:
--primary
--primary-high
--primary-high-or-secondary-low
--primary-low
--primary-low-mid
--primary-low-mid-or-secondary-high
--primary-low-mid-or-secondary-low
--primary-med-or-secondary-high
--primary-med-or-secondary-med
--primary-medium
--primary-or-primary-low-mid
--primary-very high
--primary-very low
--primary-low-rgb
--primary-rgb
Or by a custom color.