You can't create your own themes from scratch, but you can modify existing themes to make your own custom theme. You start by modifying existing theme colors and fonts:
To create a custom color theme, choose Colors→Customize Colors. Use the Create New Theme Colors dialog box to pick and choose which colors apply to text or various graphical elements in your document.
To create a custom font theme, choose Fonts→Customize Fonts. Use the Create New Theme Fonts dialog box to select fonts — one for the headings and another for the body text.
In each case, give the new theme a name and save it. You can then choose that theme from the Custom area of either the Colors or Fonts menu.
When you've added elements to your document and are using a set of theme colors, fonts, and graphics styles — even if you didn't create them yourself but, rather, used them merely to organize your document — you can use those items collectively as a theme. Choose Save Current Theme from the Theme menu, and use the dialog box to give your theme a proper descriptive name and save it. The theme you create then appears in the Custom area of the Themes menu.
To remove a custom theme, right-click it on the Themes menu and choose the Delete command. Click the Yes button to remove the theme.