Enter the options through
Tools > Options
If it isn’t checked, check the “Show all settings” box at the bottom of that window.
In the tree to the left, choose:
Text Editor > CSS > CSS Specific
Uncheck “Detect Errors”. Then, choose:
Text Editor > JScript > Miscellaneous
Uncheck “Show syntax errors”.
You can also change how it indents each type of code too, in the various settings in those “Text Editor” sub-options.