Each gizmo has several html sections, and each section has own class name. You can easily use these class names to define specific css for the gizmos.

Gizmo area:

Every gizmo is placed inside a gizmo area which is a "div" element with class name "gizmo-area". The good practice is all gizmo styles to start their definition with this class, separating it from other page elements defined in the layout:
.gizmo-area table {
    color: #f00;
    border: 1px;
}

Gizmo content:

The wrapping div for each gizmo has a class name "gizmo-content". You can use this class name to define a common css for all gizmos:
.gizmo-content {
    font-family: Verdana, 'Trebuchet MS', Sans-serif;
    color: #555;
}

Specific gizmo type:

Each different gizmo type has own class composed by string "gizmo-" and the gizmo type ("Blurb", "FaqManager", "SiteMap" etc.). Here is the list of specific gizmo type classes:
gizmo-Blurb
gizmo-NewsManager
gizmo-FaqManager
gizmo-FileManager
gizmo-Calendar
gizmo-RssFetcher
gizmo-Form
gizmo-DiscussionForum
gizmo-SiteMap

Heading:

The first line of a gizmo is its title. The titles are always placed inside heading tags as the heading number could be from 1 to 6. If you want to have custom css for gizmo titles you have use h1 to h6:
.gizmo-area h1, .gizmo-area h2, .gizmo-area h3, .gizmo-area h4, .gizmo-area h5, .gizmo-area h6 {
    font-family: Verdana, 'Trebuchet MS', Sans-serif;
    color: #555;
}
.gizmo-area h1 {
    font-size: 1.5em;
}
.gizmo-area h2 {
    font-size: 1.3em;
}
.gizmo-area h3 {
    font-size: 1.1em;
}
.gizmo-area h4 {
    font-size: 0.95em;
}
.gizmo-area h5 {
    font-size: 0.9em;
}
.gizmo-area h6 {
    font-size: 0.85em;
}

Gizmo description

Bellow the title is the gizmo description placed in separate div element which class name is "gizmo-description".

.gizmo-description {
    margin-bottom:5px;
}

Gizmo body:

Immediately bellow the description is the gizmo body which has class name "gizmo-body".
#sidebar .gizmo-body {
    margin-left: 10px;
    margin-right: 10px;
}

List elements:

All gizmos which have sub-items (News Managers, File Managers, Site maps, etc.) use list tags (<ul> or <ol>) to render their sub-gizmos. The class name of each ul/ol is "navigation_menu". The first <li> element has class name "first", the last element class name is "last". Here is an example css:
#main .gizmo-body ul li, #main .gizmo-body ol li {
    border: 0;
    background: transparent;
    margin-left: 20px;
    padding: 0;
}