You can also use the Visual Studio Theme Generator. I have only tried this with VS 2005, but the settings it generates may work with newer versions of VS as well.
In case the link goes dead in the future here is a dark theme I generated. Just put this text into a file named VS_2005_dark_theme.vssettings and import it using Tools -> Import and Export Settings:
<UserSettings>
<ApplicationIdentity version="8.0"/>
<ToolsOptions>
<ToolsOptionsCategory name="Environment" RegisteredName="Environment"/>
</ToolsOptions>
<Category name="Environment_Group" RegisteredName="Environment_Group">
<Category name="Environment_FontsAndColors" Category="{1EDA5DD4-927A-43a7-810E-7FD247D0DA1D}" Package="{DA9FB551-C724-11d0-AE1F-00A0C90FFFC3}" RegisteredName="Environment_FontsAndColors" PackageName="Visual Studio Environment Package">
<PropertyValue name="Version">2</PropertyValue>
<FontsAndColors Version="2.0">
<Categories>
<Category GUID="{5C48B2CB-0366-4FBF-9786-0BB37E945687}" FontName="PalmOS" FontSize="8" CharSet="0" FontIsDefault="No">
<Items>
<Item Name="Plain Text" Foreground="0x0000FF00" Background="0x00003700" BoldFont="No"/>
<Item Name="Selected Text" Foreground="0x00003700" Background="0x0000FF00" BoldFont="No"/>
<Item Name="Inactive Selected Text" Foreground="0x00003700" Background="0x00009100" BoldFont="No"/>
<Item Name="Current list location" Foreground="0x00A3DBFF" Background="0x01000007" BoldFont="No"/>
</Items>
</Category>
<Category GUID="{9973EFDF-317D-431C-8BC1-5E88CBFD4F7F}" FontName="PalmOS" FontSize="8" CharSet="0" FontIsDefault="No">
<Items>
<Item Name="Plain Text" Foreground="0x0057C732" Background="0x0017340E" BoldFont="No"/>
<Item Name="Selected Text" Foreground="0x00003700" Background="0x0000FF00" BoldFont="No"/>
<Item Name="Inactive Selected Text" Foreground="0x00003700" Background="0x00009100" BoldFont="No"/>
<Item Name="Current list location" Foreground="0x00A3DBFF" Background="0x01000007" BoldFont="No"/>
</Items>
</Category>
<Category GUID="{A27B4E24-A735-4D1D-B8E7-9716E1E3D8E0}" FontName="Monaco" FontSize="9" CharSet="0" FontIsDefault="No">
<Items>
<Item Name="Plain Text" Foreground="00CFCFCF" Background="00383838" BoldFont="No"/>
<Item Name="Indicator Margin" Foreground="0x02000000" Background="00383838" BoldFont="No"/>
<Item Name="Line Numbers" Foreground="00828282" Background="00383838" BoldFont="No"/>
<Item Name="Visible White Space" Foreground="0x00808080" Background="0x02000000" BoldFont="No"/>
<Item Name="Comment" Foreground="00828282" Background="0x02000000" BoldFont="No"/>
<Item Name="Compiler Error" Foreground="000000F0" Background="0x02000000" BoldFont="No"/>
<Item Name="CSS Comment" Foreground="00828282" Background="0x02000000" BoldFont="No"/>
<Item Name="CSS Keyword" Foreground="00FECE2F" Background="0x02000000" BoldFont="No"/>
<Item Name="CSS Property Name" Foreground="00FECE2F" Background="0x02000000" BoldFont="No"/>
<Item Name="CSS Property Value" Foreground="00F12FFE" Background="0x02000000" BoldFont="No"/>
<Item Name="CSS Selector" Foreground="00CFCFCF" Background="0x02000000" BoldFont="No"/>
<Item Name="CSS String Value" Foreground="0032FE2F" Background="0x02000000" BoldFont="No"/>
<Item Name="HTML Attribute" Foreground="00CFCFCF" Background="0x02000000" BoldFont="No"/>
<Item Name="HTML Attribute Value" Foreground="0032FE2F" Background="0x02000000" BoldFont="No"/>
<Item Name="HTML Comment" Foreground="00828282" Background="0x02000000" BoldFont="No"/>
<Item Name="HTML Element Name" Foreground="00FECE2F" Background="0x02000000" BoldFont="No"/>
<Item Name="HTML Operator" Foreground="00CFCFCF" Background="0x02000000" BoldFont="No"/>
<Item Name="Identifier" Foreground="00CFCFCF" Background="0x02000000" BoldFont="No"/>
<Item Name="Keyword" Foreground="00FECE2F" Background="0x02000000" BoldFont="No"/>
<Item Name="Number" Foreground="002FFE60" Background="0x02000000" BoldFont="No"/>
<Item Name="Operator" Foreground="00CFCFCF" Background="0x02000000" BoldFont="No"/>
<Item Name="Preprocessor Keyword" Foreground="00FE2F8C" Background="0x02000000" BoldFont="No"/>
<Item Name="Stale Code" Foreground="0x00808080" Background="0x00C0C0C0" BoldFont="No"/>
<Item Name="String" Foreground="0032FE2F" Background="0x02000000" BoldFont="No"/>
<Item Name="String (C# Verbatim)" Foreground="00FE2F8C" Background="0x02000000" BoldFont="No"/>
<Item Name="Task List Shortcut" Foreground="0x00FFFFFF" Background="0x02C0C0C0" BoldFont="No"/>
<Item Name="User Keywords" Foreground="00F12FFE" Background="0x02000000" BoldFont="No"/>
<Item Name="User Types" Foreground="00D64EDF" Background="0x02000000" BoldFont="No"/>
<Item Name="Warning" Foreground="000000F0" Background="0x02000000" BoldFont="No"/>
<Item Name="XAML Attribute" Foreground="00CFCFCF" Background="0x02000000" BoldFont="No"/>
<Item Name="XAML Attribute Quotes" Foreground="0032FE2F" Background="0x02000000" BoldFont="No"/>
<Item Name="XAML Attribute Value" Foreground="0032FE2F" Background="0x02000000" BoldFont="No"/>
<Item Name="XAML Comment" Foreground="00828282" Background="0x02000000" BoldFont="No"/>
<Item Name="XAML Delimiter" Foreground="00CFCFCF" Background="0x02000000" BoldFont="No"/>
<Item Name="XAML Keyword" Foreground="00FECE2F" Background="0x02000000" BoldFont="No"/>
<Item Name="XAML Markup Extension Class" Foreground="00FECE2F" Background="0x02000000" BoldFont="No"/>
<Item Name="XAML Markup Extension Parameter Name" Foreground="00F12FFE" Background="0x02000000" BoldFont="No"/>
<Item Name="XAML Markup Extension Parameter Value" Foreground="00D64EDF" Background="0x02000000" BoldFont="No"/>
<Item Name="XAML Name" Foreground="00FECE2F" Background="0x02000000" BoldFont="No"/>
<Item Name="XAML Processing Instruction" Foreground="00FE2F8C" Background="0x02000000" BoldFont="No"/>
<Item Name="XAML Text" Foreground="00CFCFCF" Background="0x02000000" BoldFont="No"/>
<Item Name="XML @ Attribute" Foreground="00CFCFCF" Background="0x02000000" BoldFont="No"/>
<Item Name="XML Attribute Quotes" Foreground="0032FE2F" Background="0x02000000" BoldFont="No"/>
<Item Name="XML Attribute Value" Foreground="0032FE2F" Background="0x02000000" BoldFont="No"/>
<Item Name="XML Comment" Foreground="00828282" Background="0x02000000" BoldFont="No"/>
<Item Name="XML Delimiter" Foreground="00CFCFCF" Background="0x02000000" BoldFont="No"/>
<Item Name="XML Doc Attribute" Foreground="00D64EDF" Background="0x02000000" BoldFont="No"/>
<Item Name="XML Doc Comment" Foreground="00828282" Background="0x02000000" BoldFont="No"/>
<Item Name="XML Doc Tag" Foreground="00D64EDF" Background="0x02000000" BoldFont="No"/>
<Item Name="XML Keyword" Foreground="00FECE2F" Background="0x02000000" BoldFont="No"/>
<Item Name="XML Name" Foreground="00FECE2F" Background="0x02000000" BoldFont="No"/>
<Item Name="XML Processing Instruction" Foreground="0x0015496C" Background="0x02000000" BoldFont="No"/>
<Item Name="XML Text" Foreground="00CFCFCF" Background="0x02000000" BoldFont="No"/>
<Item Name="XSLT Keyword" Foreground="00FECE2F" Background="0x02000000" BoldFont="No"/>
</Items>
</Category>
</Categories>
</FontsAndColors>
</Category>
</Category>
</UserSettings>