Numbers

Take a look at Sytem Preferences->Accessibility->Display. Is the display contrast slider slid to the right at all or is it on "normal"? If you don't see the slider ( ecause it might be washed out), click just to the right of "Display Contrast" to adjust it to the left. You may have to click a bunch of times to hit it. I'm posting a screenshot but it may not look right to you.

Can you describe what the "back lines" are?

I'll guess and assume you mean the table grid(?)

Here are two state of a table... unselected:

Selected:

Here, I have highlighted all the grid lines of the table:

Are grid lines what you are inquiring about?

Will you post a screenshot of your situation?

Can you post a screenshot of what you are seeing with the Google search box? Are you using Safari as your browser?

Your screenshot shows gridlines around all cells when I see it here. It appears to be something about your computer if you are not seeing them in the screenshot you posted.

Set it to Normal. Anything else and the screen washes out, including gridlines and boxes disappearing.

Seems to me this setting does exactly opposite of what it says. It reduces contrast. I see absolutely no use for it, for any disability. It only makes things worse.

Websites have also lost the gridlines? Your problem is happening in Safari as well, not just Numbers?

Please share a screenshot of the problem

What is missing?

I see grey gridlines around the cells in your screenshot. Do you not see those?