Fix: Soften multi-selected cell highlight for readability

[enklht]

This PR slightly adjusts the background colour used for multi-selected notebook cells.

The current highlight is quite strong and can make the underlying code harder to read.
I softened it by blending the accent colour with the base background.

I changed --jp-notebook-multiselected-color from:

var(--ctp-plt-blue);

to:

color-mix(
  in srgb,
  var(--ctp-plt-blue),
  var(--ctp-plt-base) 50%
);

This keeps the selection clearly visible while improving readability.
The colour after the change is also used for table row highlights, so it makes the theme a bit more consistent as well.

Screenshots are attached for comparison.
before (frappe, macchiato)

after (frappe, macchiato)