I would like flat colors, so I decreased contrast. Now ticks look gray, dim (http://i.imgur.com/T8567EU.png). If I have maximum contrast (default), they are black (http://i.imgur.com/QBHzOzt.png). Is there any way to make them black even if I have low contrast? I tried changing different color schemes, ticks do not change their color.
And one more question - can I make selections in menu darker (http://i.imgur.com/5nF1S5K.png)? With maximum contrast this selection is dark and can be easily seen, but now it is also dim. I know that I can make it the same color as normal selections, but this is not what I want.
Thank you.

You could try checking in Oxygen Settings (oxygen-settings) whether it has an option to control the shading level of the checkboxes. I don't recall seeing such an option in the past though.

Unfortunately I suspect the behaviour you are after (flat colours with high contrast for selections and various other items) isn't a supported configuration.

As I can see, there is no such option in oxygen-settings. So, I just have to get used to grey ticks. Thanks for help.

That's pretty gray, for a second I thought they were disabled.

I suspect that it's related to you color scheme, you could try to find the color and change it to a lighter one. Unfortunately I don't know which color controls the tickmark colors.

I've tried dozens of themes. For example, Zion Reversed: http://i.imgur.com/4zTpNM0.png
In any theme these ticks are dim if I have minimum shading. If I turn it to maximum, ticks may be green, white, yellow...