A way to make CSS/html and other client-side code/markup easier in Rails?

Rails 3.1 brought Sass into core. It sounds kinda like... exactly what you're looking for.

Prior to 3.1 you can still use Sass. Also helpful is Compass which provides somewhat similar functionality to Rails 3.1's asset pipeline for organizing/compiling CSS, and additionally a bunch of useful, oft-needed mixins (clearfixes, CSS3 hacks, grid frameworks, sprite mapping, and so on).

Sass gives you the ability to organize your CSS in a modular, programmatic way (or however you like, really). Behind the scenes your Sass files are compiled into CSS, which can be minified and concatenated as you wish. Sass also allows you to express the same rules more concisely, and re-use code through mixins.

From the docs:

/* nesting */
table.hl {
  margin: 2em 0;

  /* this rule compiles to "table.h1 td.ln { text-align: right; }" */
  td.ln {
    text-align: right;
  }
} 

/* mixins */
@mixin table-base {
  th {
    text-align: center;
    font-weight: bold;
  }
  td, th {padding: 2px}
}

@mixin left($dist) {
  float: left;
  margin-left: $dist;
}

#data {
  @include left(10px);
  @include table-base;
}