(Sela theme) Restoring to default

Hello there,
Here’s my site: http://mikate.ch
I wanted to change the menu colour (pink) to match logo colour (red). I did that using a simple custom CSS plugin. That was before I read that I could create a child theme, although that looked more complicated. I have changed the colour now but it is not the right shade. However, nothing I am trying to change this, is working.
I have reset my entire theme several times using a plug in, I tried switching to another theme and coming back, I have cleared any text in the simple custom css plugin.
Nothing seems to be working. The site looks plain now because I have lost all the customizations I had made, in resetting the site. I don’t want to redo everything until I have sorted this out.
Many thanks for your help.