With some color schemes, such as Breeze Classic, the current day does not really stand out among other days in KDateTable widget, a part of KDatePicker widget.

This could be considered a matter of personal preference or an issue with a color scheme except, IMO, there is a problem at the source code level.
KDateTable::paintCell uses QPalette::LinkVisited color for the current day.

First, I am not familiar with the code enough to say whether using link colors for non-link items is a misuse.
More importantly, visited links are not supposed to be prominent while the current day is.

I did a little bit of code archeology and it seems that the code has seen very little changes over many years before and after transition from monolithic kdelibs to frameworks.
In the old days, the widget used KColorScheme::ActiveText for the current day.
It was commit f17d701 in frameworks branch of kdelibs repo titled "Port away from KColorScheme" that made the change from KColorScheme::ActiveText  to QPalette::LinkVisited.


STEPS TO REPRODUCE
1. Set color scheme to Breeze Classic
2. Run an app with KDatePicker widget, for example, KMyMoney
3. Open for editing some transaction that was entered on a day other than today
4. Click the date picker button near the date field

OBSERVED RESULT
The current day is shown in a gray color, although with bold font.
The day does not stand out among other days and looks similar to inactive days (from other months).

EXPECTED RESULT
The current day is shown with a color that stands out.

SOFTWARE/OS VERSIONS

KDE Plasma Version:  6.3.2
KDE Frameworks Version: Version 5.116.0
Qt Version: Version 5.15.16 (built against 5.15.14)

ADDITIONAL INFORMATION

Note that I run Plasma 6, but KMyMoney is still built with KF5.
So the frameworks and Qt information is copied from KMyMoney.
However, I believe that the code and behavior is exactly the same in KF6.