[FEATURE] Improve font color assignment in treemap chart component

[dtslvr]

Currently the font color assignment of the treemap chart component is static (white).

From now on, the font colors should be assigned dynamically depending on the background color to improve the contrast.

• For background index 0 - 4, the font color 9 should be used (e.g. red[3] → red[9])
• For background index 5 - 9, the font color 0 should be used (e.g. green[6] → green[0])
• For background color gray[3], the font color gray[9] should be used

Extract the logic from the background assignment and reuse it for the font color assignment.

[VeerChaurasia]

hey @dtslvr can I take this one?