Using different banner images as a header above the menu

Hi, I'm just starting out and experimenting with blocks etc. 

I have created a sub theme of basis, and am trying to do this:

I want a banner-type image at the top, above the menu, and I want the image to be aligned in the top left corner of the screen, with no padding and no margin.

My css skills are not up to this but I tried changing a few things using 'inspect' in the browser, like 

margin: 0; and 

padding: 0; for 

.l-header-inner 

- which worked in the browser, but then when I put them into the custom.css for my sub theme, it didn't work.

I have been badgering away at it for hours and getting nowhere, please can someone just tell me how I can do it?

I want to create a few blocks with different banner-type images in, that I can have at the top of various pages, above the menu, where the images are flush with the edge of the screen: without any margin or padding at the top or the left or right edges. (Some of my images are left-aligned, and some are right-aligned.)

The important thing is that I want no margin or padding.

Thanks