UI color change.

[ayushi0809]

We are not able to read text due the background color. I have made few changes . Should I do a PR for the same

[nevfy-y]

The color picker could be simplified by removing the row of colors not quite usable for a particular theme. Darker colors work bad on light theme, because the text is dark, and same for the dark theme - light text doesn't work on lighter colors. There is no need in giving options that work bad.

There is another way of looking on those colors: lighter colors on the light theme are normal colors with lowered transparency. Same for the darker colors on dark theme.

When switching themes, lighter colors should be changed to their darker versions with the same hue. Normal colors (the middle row on the palette) should remain the same. Colors defined by the user should be recalculated, while palette colors could just be swapped with their darker/lighter sibling.