Advice on how to tune a Gnome Shell Theme

I am trying to customise and tune a downloaded GNOME Shell Theme.

I managed to change colours of top and bottom panels and removed the effect rounded corners. However, I was not successful to find CSS entries for other things. Is there any official manual which explains the CSS entries?

Perhaps someone can help with particular CSS? The screenshot (below this post) shows a very thin horizontal light line right under the panel. It’s hard to say which CSS causes it: border-bottom, outer / inner shadow effect, etc. I’d like to change the colour to something darker or event remove it but can’t find how.

There are couple of things I would also like to change and perhaps ask for advice if someone has time to help. I would be very grateful for that.

I would like to change colours of buttons. When I hover e.g. Activities, Applications, Places (buttons), they become black. I’d like to have them while. Also, when buttons are active (focused), their background changes to an odd gradient of blue and grey colours. I’d rather have a solid medium dark grey colour.

One least important change is about background of menu items that are hovered. I couldn’t find CSS which defines this colour. There seems to be different files responsible for background of context-menu and menu inside Applications (opened from a Top panel)

Thanks for any advises.

PS. Screenshot:

which one of all your 6 files is the theme you are now asking for help about?