If you are comfortable with cascading style sheets (CSS), these resources may be helpful for you. Please note this does not tell you how to add CSS but gives you a better idea of how to incorporate Eloqua specifics into your knowledge.

From Oracle:

• HTML email code requirements (includes HTML and CSS recommendations) 
• Customizing forms with CSS in the Design Editor (includes class names for landing pages that contain forms)
• Customizing forms with CSS in the Classic Design Editor (note this references an editor that is no longer available in Eloqua but does have some additional class names if you scroll down).
• Customizing Design Editor emails and landing pages with CSS(includes class names and sample code)

From others:

Please note these are from consultants and are not official Oracle documentation. Please use with caution.

• Code like a boss: front-end development for Oracle Eloqua (from RelationshipOne)
• The Ultimate Guide to CSS (from CampaignMonitor)

Extra sample CSS:

Code to style the "Submit" button on a form:

.elq-form .submit-button-style {
    background: #b70101;
    color:#ffffff;
    font-size: 18px!important;
    line-height:18px;
        height:45px!important;
    border:1px solid #b70101;
    padding:5px 20px 5px 20px;
    margin:30px 0px!important;
}

Code to change Form "Instructions" text formatting:

.elq-form .form-element-instruction{
    font-weight: normal;
    font-style: italic;
    color: #c5050c;
}