Hello @esthervoorpubliek-nl,

We have used the color for each pixel for the shadow that appears after the date .

So for this you have to change the color for each pixels available .

You just paste the below given CSS in Additional CSS by changing the hex color code in all pixel position according to your requirement

.custom-entry-date span.entry-day {
    text-shadow: #bea324 1px 1px, #bea324 2px 2px, #bea324 3px 3px, #bea324 4px 4px, #bea324 5px 5px, #bea324 6px 6px, #bea324 7px 7px, #bea323 8px 8px, #bea323 9px 9px, #bea323 10px 10px, #bea322 11px 11px, #bea322 12px 12px, #bea322 13px 13px, #bea322 14px 14px, #bea322 15px 15px, #bea322 16px 16px, #bea322 17px 17px, #bea322 18px 18px, #bea321 19px 19px, #bea321 20px 20px, #bea321 21px 21px, #bea322 22px 22px, #bea321 23px 23px, #bea321 24px 24px, #bea321 25px 25px, #bea321 26px 26px, #bea321 27px 27px, #bea321 28px 28px, #bea321 29px 29px, #bea321 30px 30px, #bea321 31px 31px, #bea321 32px 32px, #bea321 33px 33px, #bea321 34px 34px, #bea321 35px 35px, #bea321 36px 36px, #bea321 37px 37px, #bea321 38px 38px, #bea321 39px 39px, #bea321 40px 40px, #bea321 41px 41px, #bea321 42px 42px, #bea321 43px 43px, #bea321 44px 44px, #bea321 45px 45px, #bea321 46px 46px, #bea321 47px 47px, #bea321 48px 48px, #bea321 49px 49px, #bea321 50px 50px, #bea321 51px 51px, #bea321 52px 52px, #bea321 53px 53px, #bea321 54px 54px, #bea321 55px 55px, #bea321 56px 56px, #bea321 57px 57px, #bea321 58px 58px;
}

Hope this helps .

Best Regards!!