Improve contrast in circle counter tool

[hosiet]

This is a follow-up and improvement for #941 and #944 .

v0.8.1 provided an initial fix to the font color issue of v0.8.0 (choosing black/white font according to the color of circle tool). The current implementation is to set a threshold on the Lightness in HSL color space (threshold=75). However, this does not work well with some colors like dark blue:

Like the one in the image above, No.2 should use a white number instead of a black number.

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It's a classic problem with many discussions before. What I found:

• https://stackoverflow.com/questions/1855884/determine-font-color-based-on-background-color
• https://css-tricks.com/switch-font-color-for-different-backgrounds-with-css/
• https://ux.stackexchange.com/a/114955
• Use "L" from CIELab color space, which better fits human perception.

[chux0519]

Hi I've done this using the algorithm in first link you provide. And it looks better.