A theme in PowerPoint 2010 is a set of predefined design elements including colors, fonts, effects, etc. that gives a presentation a consistent look.

PowerPoint comes with built-in themes that can be applied to presentations.
Themes can be modified by mixing and matching colors, fonts, effects from different themes to create a unique customized theme.

Modified themes can be saved for reuse in other presentations.

How to Modify an Existing Theme

To modify a theme:

On the Design tab, select the theme you want to modify from the Themes gallery.

To change theme colors, click Theme Colors and select a new color scheme from the dropdown menu.
To change theme fonts, click Theme Fonts and select new fonts from the dropdown.
To change theme effects like shape styles, click Theme Effects and select new effects.

Further customizations like changing background style can also be done.

If the changes don’t look right after modifying a theme, try saving and reopening the presentation.

Saving a Custom Theme

To save a modified theme for reuse:

On the Design tab, click the arrow next to Themes and click “Save Current Theme”.
Give the theme a name and save it. The custom theme will now show up in the Themes gallery.

Saved custom themes can be reused across PowerPoint presentations and even other Office applications like Word and Excel.

Common Issues with Modifying Themes

Some common issues faced while modifying themes:

Text colors not updating properly when theme colors change. Saving and reopening the presentation fixes this.
Copy-pasting slides from an old presentation into a new one brings in unwanted formatting and masters from the old presentation. To avoid this, layout names must match between old and new presentation.

Best Practices

Some best practices when working with themes:

Use theme colors, fonts, effects as much as possible for consistency.
Modify existing themes instead of creating themes from scratch.

Save customized themes for reuse rather than recreating.
Ensure layout names match when copy-pasting between presentations.

So in summary, PowerPoint 2010 makes it very easy to customize themes to give presentations a unique yet consistent look. With some best practices, themes can be effectively leveraged to enhance presentations.