Changing the background colour in the content panel

Please can you tell me the best way to change the (background) colour in the content panel (= the place where I write my text and which is by default white)?

I found out a successful way by modifying the following code in the default css style sheet (https://wiki.zoho.com/_zoho/styles/dark/style.css):

.bg{ background-color: #FFFFFF; vertical-align:top; padding: 8px; }

By changing #FFFFFF to another colour all works fine concerning the content panel.

But unfortunately when doing this, the appearance (colours) of the side panel and of the Wiki name in the top panel change too (to some other colours as in the content panel).

Is there a way to change only the colour in the content panel so that the other parts remain as they are by default?

Thanks in advance for your help.