A simple solution for the "old" Opera (12.x; as far as I know this isn't possible with Blink, i.e. 15+) would be to set it to display in user mode:
User mode's default settings are designed to display webpages with
only CSS that has been specified in the options described under Site
Preferences and Style Options. By default no CSS that the creator or
site specified will be used.
You can do this globally (
Style Options..., then select your style sheet on the
Display tab, and on the
Presentation Modes tab, set the
Default mode to be
User mode), and/or use specific user styles for individual web pages (
Right-click on page >
Edit Site Preferences... >
Display, then select your style sheet.
While mainly intended for usability purposes (e.g. for the visually impaired), this should do the trick for you if you don't want any CSS to load. Whether you want to apply some basic local styling of your own is up to you.
You may also want to consider enabling
Opera Turbo if you haven't already; it loads web content on Opera's server, sends it to you in compressed format, and Opera decompresses it for you locally. Not ideal for dynamic web apps, but great for speeding up static pages on slow connections.