您好。修改FORM的Font属性就可以修改窗体和其中所有控件的字体。
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Me.Font = New System.Drawing.Font("宋体", 10)
另外，FORM标题栏的TEXT字体和颜色是操作系统决定的，要想改变，只能调用系统的api函数，截获操作系统的消息来改变。
下面的网址有C#版本的重绘标题栏范例
软糖的回答满意吗，请及时采纳，谢谢。
'新建一个VB.NET 项目，把原来所有代码删了，把下面的代码复制贴上去解OK了
Public Class Form1
Inherits System.Windows.Forms.Form
#Region " Windows 窗体设计器生成的代码 "
Public Sub New()
MyBase.New()
'该调用是 Windows 窗体设计器所必需的。
InitializeComponent()
'在 InitializeComponent() 调用之后添加任何初始化
End Sub
'窗体重写 dispose 以清理组件列表。
Protected Overloads Overrides Sub Dispose(ByVal disposing As Boolean)
If disposing Then
If Not (components Is Nothing) Then
components.Dispose()
End If
End If
MyBase.Dispose(disposing)
End Sub
'Windows 窗体设计器所必需的
Private components As System.ComponentModel.IContainer
'注意: 以下过程是 Windows 窗体设计器所必需的
'可以使用 Windows 窗体设计器修改此过程。
'不要使用代码编辑器修改它。
Friend WithEvents RadioButton1 As System.Windows.Forms.RadioButton
Friend WithEvents RadioButton2 As System.Windows.Forms.RadioButton
Friend WithEvents GroupBox1 As System.Windows.Forms.GroupBox
Friend WithEvents GroupBox2 As System.Windows.Forms.GroupBox
Friend WithEvents RadioButton3 As System.Windows.Forms.RadioButton
Friend WithEvents RadioButton4 As System.Windows.Forms.RadioButton
Friend WithEvents CheckBox1 As System.Windows.Forms.CheckBox
Friend WithEvents CheckBox2 As System.Windows.Forms.CheckBox
Friend WithEvents CheckBox3 As System.Windows.Forms.CheckBox
Friend WithEvents TextBox1 As System.Windows.Forms.TextBox
Friend WithEvents Button1 As System.Windows.Forms.Button
System.Diagnostics.DebuggerStepThrough() Private Sub InitializeComponent()
Me.RadioButton1 = New System.Windows.Forms.RadioButton
Me.RadioButton2 = New System.Windows.Forms.RadioButton
Me.GroupBox1 = New System.Windows.Forms.GroupBox
Me.GroupBox2 = New System.Windows.Forms.GroupBox
Me.RadioButton3 = New System.Windows.Forms.RadioButton
Me.RadioButton4 = New System.Windows.Forms.RadioButton
Me.CheckBox1 = New System.Windows.Forms.CheckBox
Me.CheckBox2 = New System.Windows.Forms.CheckBox
Me.CheckBox3 = New System.Windows.Forms.CheckBox
Me.TextBox1 = New System.Windows.Forms.TextBox
Me.Button1 = New System.Windows.Forms.Button
Me.GroupBox1.SuspendLayout()
Me.GroupBox2.SuspendLayout()
Me.SuspendLayout()
'
'RadioButton1
'
Me.RadioButton1.Location = New System.Drawing.Point(16, 24)
Me.RadioButton1.Name = "RadioButton1"
Me.RadioButton1.Size = New System.Drawing.Size(56, 24)
Me.RadioButton1.TabIndex = 1
Me.RadioButton1.Text = "红色"
'
'RadioButton2
'
Me.RadioButton2.Location = New System.Drawing.Point(72, 24)
Me.RadioButton2.Name = "RadioButton2"
Me.RadioButton2.Size = New System.Drawing.Size(48, 24)
Me.RadioButton2.TabIndex = 2
Me.RadioButton2.Text = "蓝色"
'
'GroupBox1
'
Me.GroupBox1.Controls.Add(Me.RadioButton2)
Me.GroupBox1.Controls.Add(Me.RadioButton1)
Me.GroupBox1.Location = New System.Drawing.Point(16, 144)
Me.GroupBox1.Name = "GroupBox1"
Me.GroupBox1.Size = New System.Drawing.Size(128, 56)
Me.GroupBox1.TabIndex = 5
Me.GroupBox1.TabStop = False
Me.GroupBox1.Text = "颜色"
'
'GroupBox2
'
Me.GroupBox2.Controls.Add(Me.RadioButton3)
Me.GroupBox2.Controls.Add(Me.RadioButton4)
Me.GroupBox2.Location = New System.Drawing.Point(176, 144)
Me.GroupBox2.Name = "GroupBox2"
Me.GroupBox2.Size = New System.Drawing.Size(136, 56)
Me.GroupBox2.TabIndex = 6
Me.GroupBox2.TabStop = False
Me.GroupBox2.Text = "字号"
'
'RadioButton3
'
Me.RadioButton3.Location = New System.Drawing.Point(16, 24)
Me.RadioButton3.Name = "RadioButton3"
Me.RadioButton3.Size = New System.Drawing.Size(56, 24)
Me.RadioButton3.TabIndex = 2
Me.RadioButton3.Text = "30号"
'
'RadioButton4
'
Me.RadioButton4.Location = New System.Drawing.Point(72, 24)
Me.RadioButton4.Name = "RadioButton4"
Me.RadioButton4.Size = New System.Drawing.Size(48, 24)
Me.RadioButton4.TabIndex = 1
Me.RadioButton4.Text = "40号"
'
'CheckBox1
'
Me.CheckBox1.Location = New System.Drawing.Point(32, 208)
Me.CheckBox1.Name = "CheckBox1"
Me.CheckBox1.Size = New System.Drawing.Size(80, 32)
Me.CheckBox1.TabIndex = 7
Me.CheckBox1.Text = "加粗"
'
'CheckBox2
'
Me.CheckBox2.Location = New System.Drawing.Point(120, 208)
Me.CheckBox2.Name = "CheckBox2"
Me.CheckBox2.Size = New System.Drawing.Size(80, 32)
Me.CheckBox2.TabIndex = 8
Me.CheckBox2.Text = "斜体"
'
'CheckBox3
'
Me.CheckBox3.Location = New System.Drawing.Point(200, 208)
Me.CheckBox3.Name = "CheckBox3"
Me.CheckBox3.Size = New System.Drawing.Size(80, 32)
Me.CheckBox3.TabIndex = 9
Me.CheckBox3.Text = "下划线"
'
'TextBox1
'
Me.TextBox1.Font = New System.Drawing.Font("宋体", 9.0!, System.Drawing.FontStyle.Regular, System.Drawing.GraphicsUnit.Point, CType(134, Byte))
Me.TextBox1.Location = New System.Drawing.Point(8, 16)
Me.TextBox1.Multiline = True
Me.TextBox1.Name = "TextBox1"
Me.TextBox1.ScrollBars = System.Windows.Forms.ScrollBars.Vertical
Me.TextBox1.Size = New System.Drawing.Size(320, 112)
Me.TextBox1.TabIndex = 0
Me.TextBox1.Text = "话说天下大势，分久必合，合久必分。"
'
'Button1
'
Me.Button1.Location = New System.Drawing.Point(88, 256)
Me.Button1.Name = "Button1"
Me.Button1.Size = New System.Drawing.Size(152, 32)
Me.Button1.TabIndex = 10
Me.Button1.Text = "确定"
'
'Form1
'
Me.AutoScaleBaseSize = New System.Drawing.Size(6, 14)
Me.ClientSize = New System.Drawing.Size(336, 310)
Me.Controls.Add(Me.Button1)
Me.Controls.Add(Me.CheckBox3)
Me.Controls.Add(Me.CheckBox2)
Me.Controls.Add(Me.CheckBox1)
Me.Controls.Add(Me.GroupBox2)
Me.Controls.Add(Me.GroupBox1)
Me.Controls.Add(Me.TextBox1)
Me.Name = "Form1"
Me.StartPosition = System.Windows.Forms.FormStartPosition.CenterScreen
Me.Text = "Form1"
Me.GroupBox1.ResumeLayout(False)
Me.GroupBox2.ResumeLayout(False)
Me.ResumeLayout(False)
End Sub
#End Region
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
If Me.RadioButton1.Checked = True Then Me.TextBox1.ForeColor = System.Drawing.Color.Red
If Me.RadioButton2.Checked = True Then Me.TextBox1.ForeColor = System.Drawing.Color.Blue
If Me.RadioButton3.Checked = True Then Me.TextBox1.Font = New Font("宋体", 30)
If Me.RadioButton4.Checked = True Then Me.TextBox1.Font = New Font("宋体", 40)
Dim style As FontStyle = FontStyle.Regular
If Me.CheckBox1.Checked = True Then style = style Or FontStyle.Bold
If Me.CheckBox2.Checked = True Then style = style Or FontStyle.Italic
' If Me.CheckBox1.Checked = True And Me.CheckBox2.Checked = True Then TextBox1.Font = New Font(TextBox1.Font, FontStyle.Bold Or FontStyle.Italic)
If Me.CheckBox3.Checked = True Then style = style Or FontStyle.Underline
Me.TextBox1.Font = New Font(TextBox1.Font, style)
End Sub
End Class
Me.RichTextBox1.SelectionFont = New Font("宋体", 14.25!, FontStyle.Bold Or FontStyle.Italic)
给点提示吧，已粗体为例（完全是我自己的方法，不知别人是怎么做的）：
'先获取选择的字体信息（字体、大小、样式。。。）
Dim currentFont As System.Drawing.Font = RichTextBox1.SelectionFont
'在重新定义某种样式的时候，不丢失其他信息
RichTextBox1.SelectionFont = New Font(currentFont.FontFamily, currentFont.Size, FontStyle.Bold)