what is the best way of Skinning A templates in Redwood Theme?

Summary:

How to do skinning on templates of redwood? and what kind of errors I might face if I do major changes?

Content (required):

I am starting to use the redwood themes but I do see that it is hard to do minor changes on skin.

e.g. I have the attached code. it is using a oj-sp-welcome-page when I use illustration-foreground image it stretches although I tried all the sizes even for 100 X 50. This is the first issue.

I used flexbox inside this template but I could not reach it from my css file or by doing overriding Please see the attachment.