Modify a default CSS color variable

Hy, I simply would like to lighten the --color-base-100 variable by 10% to use it as value for my dark theme --text-normal variable.
I don’t want to calculate it externaly and enter a hex code manually, I don’t want to externally derive hsl values of its #dadada hex value and modify them like --text-normal: hsl(h, s%, calc(l +10%)), I would like to do everything in the snippet with a css calculation referring to --color-base-100, is that possible?

Maybe not what you’re looking for, but .theme-dark’s --color-base-100 is #dadada as you know, so using this site results in a #dedede with a 10% lightening. You could try that.

.theme-dark {
    --color-base-100: #dedede;

Thank for your help but as I said