fix alerts backgrounds

[pxpm]

While working on other topic I stumbled upon this error.

The alerts background is a fixed "#FFF" color. I went to the online demo and there it worked perfectly. I dig up a big more and found out it was fixed in beta20 (we use beta19 tho).

I didn't try to update to beta20, I just added the same fix they did, by using a var() from the color pallet.

When updating to tabler20 or the 1.0.0 release we should probably remove this.