Introduce small color palette module in frontend

[MichaelBuessemeyer]

Detailed Description

In some parts of the frontend code we use the CSS color variables (and maybe also other CSS variables defined by antd). The constants could become its own module in the frontend so that one can use ColorPalette.PRIMARY_COLOR and so in the code without looking up the exact string of the variable "var(--ant-color-primary)".

Context

I just came up with this little refactoring idea while seeing @dieknolle3333 changes 9d829da