Configuration of themes and new tab page

[Chanci]

Hello everyone, I have just installed Basilisk, so I need to get my bearings. I am trying to apply a dark theme to all the pages, but only using the Swarth extension and the theme I have created with Theme Builder sometimes gives me problems with buttons (they don't look right). Is there anything that I can do to make it a little better? I have seen that on the about:config page there are some options on that regard, but I don't want to start tweaking it if it's not necessary.
And for my second question: is there a way to open new tabs on my home page? Either from the aforementioned about:config page or with some extension/add-on?
Thanks a lot to everyone in advance!