Have you wondered why the custom webpart does not follow the current themes? Well, there is a easy way to apply this; however the tricky part is to know the "Class Name".
Here is the code to load the Theme in
CreateChildControls():
String themeCssUrl = SPContext.Current.Site.OpenWeb(SPContext.Current.Web.ID).ThemeCssUrl;
CssLink cssLink = new CssLink();
cssLink.DefaultUrl = themeCssUrl;
this.Page.Header.Controls.Add(cssLink);
this.CssClass = "ms-WPBody";
Refer here:
Cascading Style Sheets Class Definitions for Windows SharePoint Services
Following are the styles that are applied for a WebPart:
Area around the menu in Web Parts.
.ms-WPMenu
Title of a Web Part.
.ms-WPTitle
.ms-WPTitle A:link, .ms-WPTitle A:visited
.ms-WPTitle A:hover
Border around a Web Part when the part is selected.
.ms-WPSelected
Contents of a Web Part.
.ms-WPBody
.ms-WPBody TABLE, .ms-TPBody TABLE
.ms-WPBody A:link, .ms-WPBody A:visited
.ms-WPBody A:hover
.ms-WPBody th, .ms-TPBody th
.ms-WPBody TD
.ms-WPBody h1, .ms-TPBody h1
.ms-WPBody h2, .ms-TPBody h2
.ms-WPBody h3, .ms-TPBody h3
.ms-WPBody h4, .ms-TPBody h4
.ms-WPBody h5, .ms-TPBody h5
.ms-WPBody h6, .ms-TPBody h6
Border around a Web Part.
.ms-WPBorder
Enjoy.
