Version of OpenTTD

20210114-master (applies to change #8557)

Expected result

The window looks good and fits stylistically with the rest of GUI

Actual result

The colors of the guide lines are too bright and irritate the eyesight.
The dark background creates a kind of internal screen that, unlike the other windows (vehicle window, extra viewport), has no frame.

Solutions

1. Changing the colors of the guides to less contrasting - more about it in another issue (Graph window - Colors of the auxiliary grid have not been adjusted to the dark background, which reduces the readability #8630)
2. Adding a frame for the dark area of the ghaph - it can be a single shadow that gives a shallow niche or a double shadow (recess) separating the graph area from the rest. The latter solution would also work for light backgrounds and the option to change the background.

Note

It is not particularly important, it's only detail, but in my opinion it gives an interesting effect, consistent with the rest of the interface.