This is my first question on Drupal answers, so be kind to me :)
I'm fairly new to Drupal, and indeed web developing in general. I have been working on my first site for a few months now, learning as I go.
In the past month, I have figured out how to make my own sub-theme of the one I was using. I have been using the sub-theme to adjust the "little niggles" that I have with page formatting directly. So far, I have been doing this by adding little snippets of css to a .css file in the sub-theme - and trying to keep this file as DRY as possible.
This has been working well for me, but when reading around, all I see is discussion of modifying/using .tpl.php files, which is making me wonder if I'm missing something and/or setting myself up for major problems as this this .css file grows?
TL:DR What are the problems and/or disadvantages of using a single .css file to make styling adjustments on my drupal site?