When beginning with CSS, it can be easy to unnecessarily extend the length of your code. This usually isn't a problem, but if you're using a format that still uses the [style] BBCode (such as blogs or subpages) and hit the DSi/o3DS 2000 character text limit or you just want to clean it up, here are some things that you can do.

• Remove spaces
Spaces are often (but not always) unnecessary in CSS, they can be removed in most cases, the same goes with line breaks.

• Putting code affecting the same element(s) together
If you're doing this, you can merge the two pieces of code using a semicolon. Similarly, you can use commas to affect multiple elements. Here's an example:

(Note that an extra semicolon is not required after !important;)
Be careful of unnecessarily combining CSS, though, as it may lead to weird consequences.

• Remove unnecessary !importants
Although !important is often needed for something to work properly (hence why it's often included in the other pages), this isn't always the case. Experiment with it and see if anything significant changes.

• Use shorter color values
When setting a color, you may be able to save a few characters by using hex colors instead (or vice versa in some cases). For example, #FFF is shorter than white and red is shorter than #F00.

• Use shorter URLs
Most sites that you may get images from, especially Discord, use very long URLs. In my experience, the best site to host images on is Imgur. No account is required to upload, the URLs are short, and the uploaded images appear on DSi/3DS.

GuiedGui's sidenote: