ScintillaNET may not support it, but as you said Scintilla does. Try this:
const int SCE_CSS_DEFAULT = 0;
const int SCE_CSS_TAG = 1;
const int SCE_CSS_CLASS = 2;
const int SCE_CSS_PSEUDOCLASS = 3;
const int SCE_CSS_UNKNOWN_PSEUDOCLASS = 4;
const int SCE_CSS_OPERATOR = 5;
const int SCE_CSS_IDENTIFIER = 6;
const int SCE_CSS_UNKNOWN_IDENTIFIER = 7;
const int SCE_CSS_VALUE = 8;
const int SCE_CSS_COMMENT = 9;
const int SCE_CSS_ID = 10;
const int SCE_CSS_IMPORTANT = 11;
const int SCE_CSS_DIRECTIVE = 12;
const int SCE_CSS_DOUBLESTRING = 13;
const int SCE_CSS_SINGLESTRING = 14;
const int SCE_CSS_IDENTIFIER2 = 15;
const int SCE_CSS_ATTRIBUTE = 16;
const int SCE_CSS_IDENTIFIER3 = 17;
const int SCE_CSS_PSEUDOELEMENT = 18;
const int SCE_CSS_EXTENDED_IDENTIFIER = 19;
const int SCE_CSS_EXTENDED_PSEUDOCLASS = 20;
const int SCE_CSS_EXTENDED_PSEUDOELEMENT = 21;
const int SCE_CSS_MEDIA = 22;
const int SCE_CSS_VARIABLE = 23;
// Reset all styles to Consolas, 10
scintilla.Styles.ResetDefault();
scintilla.Styles.Default.FontName = "Consolas";
scintilla.Styles.Default.Size = 10;
scintilla.Styles.ClearAll();
// Set colors on CSS styles
scintilla.Styles[SCE_CSS_DEFAULT].ForeColor = Color.Silver;
scintilla.Styles[SCE_CSS_COMMENT].ForeColor = Color.FromArgb(0, 128, 0); // Green
scintilla.Styles[SCE_CSS_CLASS].ForeColor = Color.FromArgb(163, 21, 21); // Red
scintilla.Styles[SCE_CSS_TAG].ForeColor = Color.FromArgb(163, 21, 21); // Red
scintilla.Styles[SCE_CSS_IDENTIFIER].ForeColor = Color.Red;
scintilla.Styles[SCE_CSS_VALUE].ForeColor = Color.Blue;
scintilla.Styles[SCE_CSS_DIRECTIVE].ForeColor = Color.Blue;
scintilla.Styles[SCE_CSS_MEDIA].ForeColor = Color.Orange;
scintilla.Styles[SCE_CSS_VARIABLE].ForeColor = Color.Purple;
// Etc...
// Enable the CSS lexer and SASS option
scintilla.Lexing.Lexer = Lexer.Css;
scintilla.Lexing.SetProperty("lexer.css.scss.language", "1");
// Set keywords
// 0. "CSS1 Properties",
// 1. "Pseudo-classes",
// 2. "CSS2 Properties",
// 3. "CSS3 Properties",
// 4. "Pseudo-elements",
// 5. "Browser-Specific CSS Properties",
// 6. "Browser-Specific Pseudo-classes",
// 7. "Browser-Specific Pseudo-elements",
scintilla.Lexing.SetKeywords(0, "color background-color background-image background-repeat background-attachment background-position background font-family font-style font-variant font-weight font-size font word-spacing letter-spacing text-decoration vertical-align text-transform text-align text-indent line-height margin-top margin-right margin-bottom margin-left margin padding-top padding-right padding-bottom padding-left padding border-top-width border-right-width border-bottom-width border-left-width border-width border-top border-right border-bottom border-left border border-color border-style width height float clear display white-space list-style-type list-style-image list-style-position list-style");
// Etc...